'There's a difference between trusting your partner and ignoring obvious warning signs. Guys I know do not send their married female friends messages signed love and song lyrics and such. And if the woman really is talking about him all the time and it's not an exxageration, then most likely there's more than just-friends feeling there whether she denies it or not. I'm willing to bet that a lot of the people who would defend the mom have never actually been cheated on. I have, and if you have this strong an inkling that something may be about to happen, you (meaning her dad) really need to plant the idea, calmly but firmly, on your almost-cheating SO that you're not having any of this, that if something happens then it will be treated as infidelity and there will be the full set of marital ramifications. You have to do it in advance, otherwise they just blankly stare at you afterwards and play dumb about the deceptiveness, and say something like "You knew I was going to cheat and you were OK it, why are you mad at me?"

All that having been said, the daughter has made the concern known to her parents, and just better stay out of it now. She's done what she needed to do, now there's nothing left to do but bide your time.