I do whatever is required to make the evening a success.
If that means plugging in to my system, then go for it.
I charge a fee that means I'm happy to do anything (within reason).
We've all purchased professional equipment, so I think the fear of 'damaging the equipment' is a very remote risk. If it's a concern, then have a proper limiter in place before the speakers.
I'd much rather run a singer through my system than have a second system (theirs) set up alongside it, that they then start taking down as soon as they're finished.