Technically speaking, you've given the answer yourself.
The general W&B limitations of the aircraft, and sometimes of each substation, are a maximum. If you exceed that maximum, you don't fly. Simples.
But the seat strength is a minimum certification limit. It's not a maximum limit. Putting somebody who is heavier than that in the seat (in sofar that's technically possible - these people are not just heavy but take up a lot of space too), unless specifically stated in the POH, is not illegal.
Common sense would dictate that you check the seatbelt can be fastened (or provide an extension) and that all normal movement of, for instance, the flight controls is possible. And since the seats are designed to withstand 215 pounds at 4.4G or so, I wouldn't worry too much about them collapsing during a normal flight.
In the event of a crash though they're not guaranteed the same protection as the other occupants. I think it would be prudent to warn them about that.
And to be honest, I would think that more-or-less the same design criteria apply to cars. And they don't hesitate before getting in a car, do they?