I'll just throw this out there, but I don't actually know how your water heating system works. All my units have gas or electric water heaters. Water heaters can have 2 heating elements and if one is not working, even if the heater is set on the highest setting, the water will not heat up fully and reheating a tank will take forever.
Have you inquired about the hot water with either of the other 2 tenants? I'd catch one of them alone or aside and ask if they're having any problems.
As far as suing you for it, you have proof that you called in an expert immediately and they determined no problem. It would be up to the tenants to prove a problem.
The other tenant happens to be her boyfriend. So he will back her up on her story. The other tenant is not around during the week and weekend. I am going to give her a call.
You are right. I have documentation that shows proof. But, just for safe measure I am going to have my plumber come out this week to check all the pipes. And ask for a complete write up of what he finds or doesn't find.
I don't trust these tenants especially these two particular tenants.