As long as flexwatt is properly wired and controlled, it is not dangerous. Think of it this way, even if you have to heat the flexwatt to 100 degrees to get the proper temps inside the tubs, that is not nearly hot enough to cause combustion. If it were, people who live in hot climates (like Mesa, AZ) would have their houses burn down every summer. If your BF thinks of it this way, he should understand it is not something to be heinously concerned over.
There is always the possibility of something failing, but there are plenty of other places in your house where this is true as well.