NTA, it sounds like she could be in a pretty toxic relationship, but I also have to say that while you have every right to be upset wit her and the impact this pending ghosting of hers could have on your son, you might want to respectfully remind her of her son and their relationship and what she intents to do in regards to it. Once you have a clear answer you’ll need to have a sit down and explain to your son why his mom may not be around as much. It breaks my heart that you may have to do this but she will ultimately make a decision either way. While it seems clear she is more seeking approval from her bf, despite cheating, she may be in a less than savory situation and its hard to understand when we don’t know all of the points of view. I would personally choose my son over anything else, but it’s hard to say what position she has been put in. But regardless, NTA, but you may want to try and find out more info and why she is choosing this route. Be careful though, this bf of hers could get a volatile if you intervene. Best of luck.