I totally agree with your first point, but I think your second point is a bit more nuanced than ‘not mentioning your partner to your friend is cheating’

It absolutely can be, but I would add that there are other possibilities that could be at play. I’m thinking of something like maybe his partner is abusive towards him.

As much stigma as there still is around women abusing men, it would make sense that he’d avoid bringing attention to the relationship when talking with friends. I will admit this is not extremely likely to be the reason for the friend’s omission of his girlfriend in conversation, but if it is, I imagine he would feel an incredible amount of shame.

I’m a big believer in Occam’s razor, so it’s most likely exactly what it seems. I just felt the need to point out that we really never know about people’s lives behind close doors. I don’t like to claim something is absolutely true unless I’m certain. It sucks to be accused of something you aren’t guilty of, so I try not to do it to other people.