How does Islam balance avoiding spying with addressing lies in a relationship?

Assalamu alaikum. I need some clarity on an Islamic perspective regarding relationships and trust. I was seeing a Muslim man, and I really cared for him, but something came up that made me uneasy. Long story short, he spent his birthday weekend with another woman. Here's what happened: I noticed a woman interacting a lot with his posts on different social media platforms. I looked at her profile and observed for a few days. On his birthday weekend, she posted about doing an activity he had told me he always wanted to do. He said he couldn't spend that weekend with me because he was working. After the trip, he apologized for being out of touch and said that on his work trip, he ended up doing that activity. He sent me a picture, and it looked exactly like the scene in her pictures. It was obvious he wasn't where he claimed and was with her. I asked him about it because it was so clear. He denied it. I didn't accuse him or mention her; I just asked, "Is she your girlfriend?" At some point, he said, "This is haram. I can't talk about her." I looked into why he might say that to understand better, and I learned Islam has guidance about suspicion and spying. Did I do something offensive? How does this fit with relationships? If I'm not supposed to be suspicious or spy, but I see something clearly questionable, was I wrong to ask? At this point, I'm just curious. I think the relationship is over because I suspect he spent the night with her again. So, this is more for my own knowledge. JazakAllah khair.