a community of practitioners
I have a spray bottle with warm water that I spray on the client and remove the scrub or mud with towels warmed up in the microwave. I use the hot thermal shells carried in Massage Warehouse rather than stones. They are self heating and lighter to carry. They do add an expense though @ $6 a treatment when using 2 shells. I do all muds/scrubs on the table. I do not send them to the shower for exactly the reason your friend pointed out. I did this once and had that happen. What was suppose to be a 90 minute session ended up being almost 4 hours! And the person did not compensate me for my total time.
As for damages, I carry extra insurance for this. Talk to whoever you have your home/renter insurance with and explain what you need. Mine is now through Farmers, though I was originally with State Farm. Most major insurance companies will have a policy for damages to a customers property.