Really, I don't know how the M/s thing works, but I would think that such things as this should be discussed prior to enterting into a relationship. If he had told her he was going to see/play with others, it should have been her choice whether or not to continue with him. And he's telling her there's no one else.
Lack of trust on your part, mic? Perhaps! However, there's a lot of players in the world. I once did something very devious to learn whether or not a Dominant was for real or not. I learned that he wasn't. He was a player and one of the biggest liars I had ever met, and damn good at it.
Was what I did ethical - probably not, but I saved myself a lot of future heartache so, to hell with ethics!