I agree that it may turn into a sexual thing. And, I agree that you should confront her before it goes any farther. However, what she is going to hit you with is your snooping and the fact that nothing has been sexual and that they are just friends and she had to have somebody to talk to. So, what arugment will you have then?
That's his call, of course, but I would simply say "You and I both know that's simply not true. Please stop lying; it's incredibly disrespectful."
There shouldn't BE any "argument." He should simply let her know what his boundaries are, and that he's willing to do whatever he needs to do in order to protect himself.
