I'm sorry you found out like that and no one had picked up on it. I've had the opposite happen, they would wind me up thinking my ex is with someone else but I would find out it was not true. None have ever said he was happy though, they always said it was something superficial that would end soon and he would be back with me. I feel like this is a pretty scripted thing some of them say just to cover their bases just in case you do find out a guy is seeing someone else. Then if you know the guy is actually with someone they are probably not going to say yes he's happy with them and not planning to end it.
When I asked about a guy who is married - who I unexpectedly had strong feelings for but really did not want to be with - they kept telling me oh he's not happy with her, the marriage won't last, they are like roommates, they are like brother and sister, they only got married because of family pressure, he didn't want children because it would make things more permanent between them, he wasn't sure if he wanted to stay married, and so on. And I could definitely believe some of what they said because the guy would complain and let it be known he was really unhappy about being married and that he was no way ready to have children. And he would take his ring on and off his finger constantly. But 2 years later they are still married, and one day it may indeed end but he is already long gone out of my life.
I'm sorry I know this isn't exactly what you were asking. I do think some readers would be honest and tell you that if that's what they see. But they have to see it for themselves to really know. If they are only going by what you told them then it's really questionable how they see that relationship. Just going by my own experience most readers would rather talk about possibilities rather than just close the door on something and tell you to move on. I have even asked idly about someone I had no interest in just to see what they would do and they take off making it sound like there's some great potential for a romance.
I have had a few readers just tell me to move on, not because my ex was in a relationship but because they thought he would never change or that it would take a long time for him to come back and by then I wouldn't want him anymore. Again I feel like this is something scripted they say that will hit the mark with a lot of callers, and I think they cross the line if they tell you what to do (unless you ask their personal opinion). IMO their job is to report what they see and let me make my own decisions. But the thing is they have to be able to see it. If they don't pick up on something without me telling them then how can I trust anything they say about it? Kind of shady too if they don't apologize or acknowledge it if they missed something.