To elaborate on that "something"...

Unpopular opinion, but Greg and Victoria both have experienced the loss of a parent. When people have experienced similar types of suffering, there is an unspoken "familiarity" and understanding that creates a sense of closeness. Now, this is a stretch, but I can see how that "familiarity" could be perceived as emotional cheating initially, before their "rekindling" evolved into what we see today. I could also see how Johnny/Greg's ex could be jealous of what they don't understand, too, which from their perspective is valid.

Is it messy and complicated? Absolutely. But no one really knows what happened between these people aside from them, and even from that vantage point, they will only know from their own perspective. (I.e. Johnny will only know from his perspective; Greg from his, Victoria from hers.)

It's a "He said, she said" situation, which the truth is probably in a little from Column A, and a little from Column B. I personally don't like making judgements about people without having all of the info, and I'm not sure if we are going to get all of it.