Nowhere, I hope.
I just never pegged Riker as a cheater. Especially now that he's married to Deanna.
Not going nowhere (pardon the double negative) doesn't necessarily lead to an affair. That circumstance would require some similar non-platonic feeling on Riker's part, which I didn't get the impression he had based on the tone of the scene. I also think it has a lot more to do with Vale's own emotional distress than anything. It's also possible that Riker saw something that wasn't there (or was something else) because of his own maligned state of affairs with Deanna.
Another facet to this is that, iirc, with the exception of Jaza, Riker is the closest friend Vale has had on Titan
. Troi, who might also fall into that category, is not really in a state to help anyone. So, this fact might also play into her reaction.