To some extent, this is a question of logic and film interpretation. I think you're drawing a hard binary where your friend is proposing a spectrum. Yes, it was uncommon for men to touch the bare skin of women they weren't related to; yes, young people touched each other illicitly, and Mr. Darcy may have even had sex while he was at university. (It would have been with a woman who sold sex, probably.) But even if he had literally ever touched a woman's bare hand before, that doesn't mean that in the context of day-to-day life, it wouldn't register as a bit of a shock to him to do so, particularly since he's attracted to her by this point.

As I wrote in this previous answer on premarital sex:

Fiction, however, did not reflect this reality. Illegitimacy and pre-/extra-marital sex were certainly acknowledged in eighteenth century fiction (though they generally would not be in fiction of the nineteenth), but heroines tended to be strictly virginal. In part, this has to do with different standards for different social groups, as you suggested in your question text - the populations studied above are largely made up of the working classes, with individuals of the almost-upper-class social stratum of Jane Austen heroines (the landed gentry, the clergy, and the lowest nobility) representing a small portion of the whole that wouldn't greatly affect the percentage. Even the daughters of London tradesmen, solidly middle class, were said by social reformer Francis Place to have been less than perfectly chaste in the late eighteenth century without incurring social stigma. Where there was any pretension to gentility, though, virginity before marriage was the rule, which would include the middle classes by the early nineteenth century as possessing refinement became more and more possible.

One also has to note that Jane Austen's heroines, and heroines in general, were not in the specific situation where premarital sex was kinda-sorta permissible: the engagement was typically the end of the story. An eighteenth/early nineteenth century heroine was usually in the position of sorting through suitors to find which one was sincere and worthwhile, and protecting her virginity from those who intended to take it by force or trickery. Even a heroine who came from the peasantry like Pamela Andrews (of Pamela) or Charlotte Summers (The Fortunate Parish Girl) would remain virginal, because the standard for heroines was to model exemplary behavior for readers, a disproportionate number of which were of high enough rank that they expected to remain virgins until marriage, and they also were on the defensive rather than secure in planned marriages.

In adolescence, young men and women of the gentry and aristocratic class did not see that much of each other. They were educated separately, either at home or at a boarding school; girls typically "came out" into society as courtable adults around seventeen, while boys would continue on at university or on a tour of Europe, not being considered proper adults until into their twenties. It was acceptable for these young men to be discreetly affectionate with young women of a lower social status, but as stated above - young ladies had to be virginal, and particularly could not be seen consorting with footmen or farmers' sons. An adolescent gentry girl might only see a non-related adolescent boy of her class if her brother brought a friend home from school on a holiday, or at church or in a shop. It would be silly to argue that nothing sexual ever happened until an engagement, but it would be equally silly to argue that the modern (20th-21st century) adolescent experience of romance and sex is universal. It's quite plausible that Mr. Darcy was mainly used to touching an adult gentry woman's gloved hand, unless she was related to him.