I am not aware of any cases in which an armourer was sued by an injured wearer of the Armourer's product.

Part 1: Context

So, some background. I am going to talk about 15th century England, because that is both the golden age of plate armour and a period when what we call the common law was more professionalized and codified than it was earlier in the Middle Ages, and when lawsuits and court cases had mostly replaced older forms of settling legal disputes (like trial by combat). Mind you, I am not a legal historian, I am a student of arms and armour history and so will talk about the relationship more from the Armourers side than the lawyers, as it were.

So, first of all, who are our parties in this hypothetical dispute? On one side, we have a Knight. This is a titled member if the aristocracy; at this point, an elite minority within the elite minority of (generally) landholding, full-armour-clad 'gentle' people (a heterogeneous class of titled and untitled people which had fuzzy borders). This is to say Knights were not numerous - on the battlefield they would be greatly outnumbered by untitled men at arms and off the battlefield by untitled gentlemen. So if someone was a Knight he was powerful and wealthy, and almost certainly influential - having both untitled gentlemen as his clients and peers as his patrons.

On the other hand, the armour would be made by an armourer - not a blacksmith, but a skilled and (if he was making armour for the upper aristocracy) well paid specialist artisan. Armour for a Knight night be made to order and to measure; while most armours worn in England were cheaper imports from Lombardy, the elite of the elite wore custom armour, often made by boutique Armourers in London or perhaps Armourers in the low countries (they might also order from Italy). So our Armourer, while lower in standing than a Knight, was not a poor man. He might live in London or abroad, and would be a member of a guild (with its own charter and privileges), unless he was in an area without guilds like Milan.

I'm hoping a legal historian can add to my answer (I'm merely a humble lawyer). I'm going to rely heavily on Richard Epstein's casebook on Torts, which dives a bit into the history of them. Civil suits in England required an individual to plead one of two forms: Trespass and Case. Trespass was for direct harms caused by the defendant (he punched me, he entered my lands, etc.). Case is for indirect harms. An illustration of the difference was given in Reynolds v. Clarke: "If a man throws a log into the highway, and in that act it hits me, I may maintain trespass because it is an immediate wrong; but if it lies there I tumble over it, and receive an injury, I must bring an action upon the case; because it is only prejudicial in consequence, for which originally I could have no action at all."

But both of these deal with an intentional act (throwing a log), what of negligence? The problem can be a bit opaque because of the way people plea their cases. In the famous "Thorns Case" (Hull v. Orange) of 1466, a plaintiff alleged that the defendant "by force and arms broke the plaintiff's close" (essentially, entered upon his property). Specifically, the defendant cut a thorn bush and the thorns fell on the plaintiff's property. The defendant responded by saying that it was an accident that the thorns entered the plaintiff's property. The court held that it didn't matter if it was an accident as long as it was caused by an intentional act. However, according to Professor S.F.C. Milsom, at least one of the Judge's opinions suggests that the issue was really the way he phrased it (if he blamed the wind or said he did everything in his power to prevent it, then there could have been a trial. Perhaps, at trial, the defendant's negligence would have mattered?).

Another problem is the way the pleas had to be alleged. They had to allege that someone acted with force and arms against the King's peace. That's a very funny way to allege malpractice. Milsom gives an example that would mirror the one you're talking about here. He gives the example of a smith who did his work so badly that the horse died. Rather than say "the smith committed malpractice," he would say "with force and arms the defendant killed the plaintiff's horse, to his damage and against the king's peace." According to Milsom, the defendant would plead not guilty and the actual trial would determine whether there was carelessness done.

All this is a long-winded way of not actually answering your question. But it hopefully steers you in the right direction of how tort law worked around this time (I think the oldest case cited was 14th or 15th century, though). Essentially, some of the specifics have been hidden behind the veil of the forms of action in England, which required alleging specific intentional acts. Given that, it would be hard to find an allegation that poor work led to injury.

I am only aware of Blacksmith malpractice in London, so ill attempt to answer your question.

The Blacksmith Company in London had a legal right to set regulations and standards for blacksmiths in the City of London. They did not have the right to confiscate sub-par products but had the right to seek out and break them if they were within 4 miles of London. The Company also fined blacksmiths for poor workmanship and had the power to ban the work of Blacksmiths who consistently produced low quality work.

So a Knight in London sold sub-par armor by a Blacksmith can appeal to the Blacksmith Company of London to have the Blacksmiths inventory inspected, broken and the blacksmith fined and kicked out of his profession.

Source: The Country Blacksmith by David L McDougall 2013.