Hmm.. she was wrong to make the bet without informing you, certainly. She should honour the bet, too.
You should tell her to obey this friend, but tell her off and warn that she's not going to get away with such behavior.
In the way of being less self concious, maybe tell you how you feel about that, and perhaps you could arrange a little punishment with the friend in question, since he seems to go in for a bit of kink.
She didn't know what she was doing was wrong, I suppose, so go easy and tell her how to act from now on.
(how about doubling the amount of time for her being a french maid?)
This sounds like a nice opportunity to me!