While I 100%....no, 1000% agree that you have ZERO obligation to loan her the suit at all in the first place, I think at this point the best option is simply to let her take it home next time, fully accepting that you may never see it again. One of two things will happen. She will wash it and bring it back, which is no different really than what is happening now. Or she will never bring it back. In which case when she "forgets" you have the perfect out. You say "Oh that's too bad, that's the only spare I had. I'll see you in an hour." And go on about your workout. Just never bring another one. I dare her to say "why haven't you bought me another suit yet?" I mean really. She will be forced to remember to bring it then, because you won't have one to lend her and it's her OWN fault that you won't have one to lend her!
It would be worth the price of whatever the suit cost in order to never have this conversation again, to me.