There is some really poor advice in this thread from people who "think" they know what they are talking about.

First, whether there is criminal or civil liability is a matter of local law which may apply differently to any given flight, e.g., ORD-MIA might be different than ORD-LHR.

Second, unless someone can point me to it, I have never heard of any protection for a citizen assisting another citizen, e.g. crew member while there are some states which would protect a citizen assisting law enforcement, e.g. FAM.

Whether courts exercise "common sense" (a court's job is to obey the law not what you happen to think) may also be tempered by the question of whether the individual suffered real damage and whether the individual caused the situation and the assistance might be self-defense.

All of this aside, this is simply a risk which people tend to take. If you have a crystal ball, you look into it and it tells you whether the individual is harmless or intends to or may cause a risk. But, if you don't, ties go to safety.

Bear in mind that even if you do win in the end, you are likely to need your own lawyer and the cost of those legal fees could well exceed anything you might avoid paying the individual.