The idiom 'fair game' means, legitimate target or object. For example, if Ms. A's husband were doing the same thing with someone else, then Ms. A's doing it with someone else is considered fair, legitimate, given her husband's behavior. But given the limited context above, I'm not sure what 'a fair game' is supposed to mean.
Seems to me it could be synonymous with "an innocent game"? I'm confused, sorry. Do you have more context?
By the way, who is "Ms. B"?