In general the PIC has an obligation to enforce some safety regs, but not others.

For example, pax are required to wear their seatbelt at certain times, and the PIC is required to inform them of this...but he is not required to FORCE them to comply, or to terminate the flight if they fail to do so.

The PIC can also get in special trouble if the airplane is used to do certain things which are specifically addressed in aviation regs, like drug transport. He can lose his licenses even if criminal charges are not brought to bear (admin law).

For other non-safety and non-aviation related illegal activity, the PIC has the same obligation as any other citizen confronted with illegal activity...he could be an accessory, especially if transportation across state lines is involved.

In the event of a post-flight DUI incident...an airline CA might be held responsible (or at least sued) because he is in charge of the bar, so to speak. It would be hard to prove, because the CA's duties do not involve supervision of alcohol distribution, and his flight duties actually preclude him from doing any such supervision (he cannot hang out in the cabin post 9/11)Actually there has been at least one such case, but I recall the airline, not the pilots getting sued.

A corporate pilot could probably not get sued successfully unless he personally served the booze. Normally a flight attendant does that, and the pilot and FA both work for the owner...if the owner says "give me another beer" to the FA, the pilot is pretty much out of that loop.