@Bex
Ask them to replace it before they leave.
I had a set of guests that broke an Ikea plate, not only did they tell me about it, but they went to Ikea off their own bats and replaced it, leaving me a small present to boot.
Using equipment for purposes it was not designed for is not only dangerous (fire hazard) but down right careless, I'm generally laid back, but this is an extremely serious matter and should not be brushed under the carpet.
At the very least these Guest should get a thumps down or more correctly be completely banned from Airbnb they could have burnt you listing down.
Regards
Cormac
The Explorer's Club Krakow III