It’s weirdly beautiful and precious that you two share that level of intimacy, and because of that, your responsibility here is significant. I know you have emotional and financial investment in this, but tread very carefully. It sounds to me like you want your daughter to brush this off, and she well might, but if she doesn’t, you don’t want to be pushed into a corner where you urged her to stay with a man she no longer trusts. That’s a corner that’s hard to get out of, one where it could be construed as not putting her happiness over the tidy story of a wedding, a lovely husband and a nice house.
But, since we’re here, let’s get to it. Did he cheat? Yes.
Not because of what bits of what went into which parts of who, but because your daughter was worried this might happen (which is telling, she obviously knows what happens when he goes out with his friends), she expressed to him that she would be really uncomfortable and upset if it did, and he did it anyway.
If your daughter had said to him, ‘It’s your buck’s night, go wild, do what you want, I don’t want to know,’ then that’s a different story. One person’s dildo slip is another person’s text with a workmate. It’s not the actions themselves that make it cheating, it’s the disregard for our partner’s expressed fears, feelings, insecurities, whatever you want to call them. That’s the bit that hurts. And that’s the bit that makes you look sideways at someone and go, “hmmm, that’s not who I thought you were. Will I be safe with you?”
That’s what I think your daughter is doing, as she “spirals”. She is adding this version of the man she loves up with all the versions of him and deciding if he’s still the person she wants to build a life with. And she has every right to do that.