The fact that you've already heard about it makes me think that it's only a matter of time before everyone else finds out. Once her XYZ sisters at Private U find out about her XYP affiliation, they'll probably have no choice but to nullify XYZ membership because of the NPC unanimous agreement forbidding dual NPC memberships.

Not only that, but she'll most likely catch a lot of crap from XYZ for even being dishonest with them.

As cuteASAbug said, there's no real method in catching girls who do this, but people do find out.