We might be needing to back the truck up on this whole premise.
laying hands on a nobleman is probably a class offense
trying to arrest anybody by yourself is plain dangerous
why didn't the commoner just report it to the authorities
Since this is basically a D&D forum, and though its inspired by medieval things, it has no obligation to be historically accurate.
So, assuming a PC is either the Commoner or the Noble, how can we make this situation appear authentic and generate some interesting game drama?
I have no doubt a PC could physically arrest somebody, odds are good they are better at it. So let's leave that part alone.
a pseudo medieval legal system would probably not be exactly like a modern system, but could share elements.
The witness (and that's what you really have here in this case) would take the claim to the authorities. If there is other evidence, then there could be a case. If its just one man's word, the station of that man would probably have some bearing on its merit.
I see 2 styles of courts: Noble run, and appointed judge run. Noble run basically means the noble has nothing better to do than preside over cases/complaints. I assume, at some later point in history, dedicated judges were given this job, freeing up the nobles time.
While real law probably has some historical conflict of interests, you could probably rule that cases get tried at the authority level above the station of both parties. Thus, local small noble would get tried at his boss's court. He wouldn't be trying it himself.
That still leaves room for corruption (his boss might like him), but sets up the semblance of "fair" trial. This would seem a logical extension of even 2 commoners having a disagreement. They go to the Noble and have him decide it. In this situation, since the noble is part of the dispute, you go a level up.
That's just setting up the premise of how the case would be decided (which further details could be worked out for flavor, maybe each side puts stones representing facts on a scale).