I would argue that your friend's position is out of date. It's true that historians like Thompson, Hill, and Hobsbawm (I would also add Lawrence Stone and maybe Robert Brenner) argued that the Civil War was caused by economic issues. Stone's Crisis of the Aristocracy was probably one of the latest and most forceful expressions of this position, and this came out in 1965. The more recent work beginning in the 1980s reasserted two key elements that discredit the economic approach you suggest. The first is that the long term structural problems in England were less significant in causing the outbreak of warfare compared to the immediate events c. 1637-1642. Stone, in his Causes of the English Revolution calls these the “preconditions” and include the rising economy, booming population, perceived increase in partisan politics, and Elizabethan religious settlement. Even in this expression, the economic element is in the minority. Far more significant in initiating hostilities between Parliament and Crown were the loss of the Bishops Wars, the perceived tyranny of Charles I in his financial extractions, the Irish Revolt of 1641, and Charles’ political miscalculation (such as the attempted arrest of the 5 MPs in 1642). It is also worth mentioning that in the prosopographical studies of the participants in the war, there is no firm economic position for either side. Mary Keeler’s work on the members of the Long Parliament found no strong correlation between wealth and their alignment in the war.

The second element argued consistently for the last few decades was the primacy of religious belief in the causes of the Civil War. While it is not a perfect identifier, most historians agree that those more likely to be sympathetic to Scottish Presbyterianism, identify with the ‘Puritan’ group (a poor label, but one we are often stuck using), or belonging to the “independent” churches were all the more likely to be for Parliament. Those who sided with Charles and Laud on the function of ceremony in the Church, or even tended to be more neutral in the religious controversies of the 1630s tended to be for the Crown. The Marxist historiography of the 1940s to 1960s tended to see religious belief or conviction as a false front for more self-interested economic or political motivations. The abundance of writing that followed has reasserted that religious politics were in fact about religion. The same goes for the English Civil War; the immediate causes of conflict were thick with religious meaning. Here is a rough outline of the religious causes of the Civil War.

1630s – Laud and Charles implement new standards for Parish Worship including removing the “communion table” from the center of churches to naves, and the tables were to be placed altar-wise with sufficient cloth and decoration. Altar rails were erected to stand between parishioners and the priest.

1637-9 – Charles attempts to install a Service Book in Scotland, similar to the Book of Common Prayer used in England. Though Charles consulted leading Scottish Divines and nobles, a significant proportion of priests, elites, and middling Scots felt that the service book contained too much “popery”, mainly being ceremonial aspects of worship. James I had tried to implement such a book, but work had halted by 1621. When the book was rushed to be used (most people and priests never had the chance to read it before it was required to be used in service) riots broke out in Edinburgh. The conflict between Charles trying to implement his service book and maintain his authority over Scotland caused armed conflict to break out. Charles sent an army north into Scotland and was defeated handedly by the Scottish Army. The Covenanters (as the Scottish rebels were called) wanted to reduce (or even remove) Scottish Bishops from power, restore power to the individual Presbyterian kirks to decide their religious expression, and to remove an English (or papistical) prayer book.

1640 – After Charles had been handedly defeated, he summoned a Parliament to give him the necessary funds to end the Bishops war, but the parliament refused. This was for two main reasons – the first being a list of ‘grievances in need of redress’ typically associated with the excesses of the Personal Rule (whether Ship Money or the maiming of Prynne, Burton and Bastwick). The second was that many MPs were sympathetic to the Covenanters cause. The Parliamentarians had no desire to pay for a war already lost for a cause they did not support, and this was absolutely a war about religion and the authority of the British Crown to establish religious homogeneity in its dominion. Charles was convinced he not only had the authority to establish religious law in his kingdom, he had the obligation to do so. The first parliament called (the Short Parliament) was dissolved after a few weeks. Charles was forced to call another Parliament, perhaps even more hostile to Charles’ religious goals.

1641-2 – The Irish Rebellion. In 1641, a group of Irish Catholics lead by disposed Old English Catholics in a raid against the Ulster plantation. Perhaps as many as 10,000 people died in the initial rising (Ó. Siochrú, “Atrocity, Codes of Conduct…” P&P 2007). The reports of the slaughter of Protestants in Ireland were highly exaggerated, with some publications in England claiming 200,000 Protestants had been slaughtered. The conflict between the Catholic majority and the Protestant minority caused panic in England. Parliament feared that the Catholic rebels would invade England, and that Charles was not taking action against the Catholics. As the relationship between Charles and Parliament deteriorated, Parliamentarians feared that Charles would import or support an invading Catholic army in England to restore his authority.

While the above may not fully account for every aspect of the Civil Wars, it should make clear that the initial conflicts (mainly the Bishops War) were founded upon disagreements over religion and the Crown’s ability to establish and prescribe correct religion. Economically speaking, yes there were many Parliamentarian merchants and men who had invested in overseas ventures (Plymouth, Massachusetts Bay, and Providence Island). However, their collective actions has as much to do with religion as economics. The conflicts of the Civil War were not about with whom merchants could trade, or about establishing a for profit (capitalistic) enterprise. Land owning gentry were as much capitalists by that definition as the merchants. It’s also possible, as it was for the Quakers a generation later, that merchants preferred to do business with other men of the same religious outlook. In other words, being a Puritan didn’t mean you were more likely to be a merchant for ideological reasons (the Puritan Ethic), but instead it was because people of the same faith tended to do business with one another, trusting each other in the same way they trusted each other as dissidents of the English Church.

And as a last item, one of the more understated elements in the English Civil War was the role of neutralism. That being the significant proportion of the population that did not want to take sides in the conflict. Rather than being a class of agricultural workers and artisans supporting Parliament, there were large groups in counties like Somerset and Devon that resisted all attempts to bring them into the war. These “clubmen” (named for their most common weapon) would meet armies on the outside of town to try to prevent the seizure of food and supplies from the town, and would stand against both Parliamentarians and Royalists.

Religion was a central component of early modern English life, and was absolutely the driving force for most participants in the war. Perhaps Oliver Cromwell stands as the best example, as a man who suffered from debt at an early age, but seems to have avoided economic gain during the Civil War. Instead he focused on making sure his soldiers were allowed to worship in their independent churches, and hear the sermons from the radical preachers that followed the army. Cromwell himself felt he was one of the elect, chosen by God to be saved, and to lead the fight against a blasphemous king who, in Cromwell’s eyes, subverted the true faith and tried to instill Catholic beliefs in the people at large. Cromwell was not a rising bourgeois trader or merchant – he was a country gentleman who had experienced a religious awakening, and was determined to see God’s work be done.