If this has been going on for a few years, don't you think the mother has some idea that SOMETHING is going on? I ask this simply because I think if it's been going on that long she suspects something. Do they have a strained relationship or is the mother and daughter very close? Maybe she is in denial and doing it face-to-face has got to be hard, but maybe if you show her you know it'll make her wake up and realize the reality of the situation...

I hope everything works out whatever you do. You're a great mom, don't forget that