I found out my partner has a "thing" with a former lover of his, and I'm not sure it's a healthy situation

I'm in a poly relationship with a guy, we talked about his other partners. Fast forward to months later and I'm talking to him about a former lover, encouraging him to talk to her despite my terror/insecurity of his feelings for her. But he's comfortable with his heartache about her. She's married to a monogamous man and happy. I asked questions about them and their history. During this he casually said they occasionally have a Thing via the internet. I wasn't happy about it but didn't berate him, only stated clearly that he should have told me that when we discussed metamours. But I'm bothered. I'm wondering now why he didn't tell me then. Her husband is why they broke up after trying poly. But now I'm wondering if he knows about them having their occasional Thing at all. If not, I'm not okay with that. But I don't know what to do. I'm still not sure what I want to do. If they're cheating I can't condone it. I don't want to leave him. I'm just feeling lost and heartaching.

The main issue here is that your partner is doing something that seems sketchy or not entirely above board here, because A.) he didn’t disclose to you the whole story about this former lover when you first talked about her, and B.) you’re not sure whether this is an open and consensual polyamorous arrangement for all parties involved.

This is something to bring up with your partner: “In order for me to feel like my polyamorous relationships are safe and healthy, it’s important for me to that the extended network that I’m involved in has a shared commitment to openness. This “thing” that you have with your former lover - can we talk about the terms of that? Does her husband know? It also bothers me that I didn’t get the whole story the first time we talked about her. Can we revisit our expectations and commitments around honesty and openness?”

It’s up to you how you react to his response. If he insists on continuing to have a relationship that you don’t want to be party to, even as a metamour, then you have to decide whether that’s a dealbreaker for you.