Well you already snooped, but you can’t really bring anything up without admitting that.

Might just have to say something like “hey _____, I went through your phone and noticed how you were talking to _____. Is there a reason your telling her fake things?”

Could backfire massively if she takes this as an invasion of privacy, which it was, and try to say that you shouldn’t know that stuff anyway, you shouldn’t. But there is basically 0 other way to find out unless you talk to the other women.