Yes, I think so (and even if it wasn't while they were still together, they're not divorced so legally, yes he has). My dad walked out and then said a year later he had a girlfriend for a year, so it was probably the reason for him leaving, although he won't admit it. I never realised at the time, never even doubted it or considered that he may be having an affair- none of us did, but looking back now three years later it all seems so obvious- being absolutely awful to my mother, working away several days a week, working very late nights etc. OP, it's a horrible situation to be in, as I'm sure you know, but after the first while, it does honestly get a lot easier. You have to support your mother's decision and try to move on from there- if she wants to stay with him, try and respect that. My parents did that for a couple of weeks and it didn't work out, but then again, it was all my dad's decision and my mum was just desperate for him not to leave- but those few weeks were a mess- basically he wanted nothing to do with my mum- wouldn't speak to her, wouldn't let her cook for him or wash his clothes, and he was basically just living there for me and my brother and sister, so in a way, it may be better that your family is trying to carry on as normal, because as hard as that is, it's a very understandable way of dealing with the situation. Although, you and your mother shouldn't just let him get away with it and pretend like nothing ever happened- he has to know how hurt you both are, and you have every right to be annoyed with him. But if he genuinely is sorry and won't do it again, I'd say give him a chance OP. If your mother is really dependant on him, it would be better that way. I'd definitely recommend talking to a friend about it. I never did, and to this day, never have, but definitely regret that. Talking would probably help. Good luck!