Is it normal to have been ditched?

once upon a time in our friendship group, there was a couple. A couple of no less than two years strong. One day, the girl started texting another boy. She started ignoring her boyfriend and spending time with the other guy. In no less than a week, the girl had decided that the other guy had more to offer and more love to give. She broke up with said boyfriend, yet did not tell him why, leaving him to grieve over the holidays, whilst the rest of us supported both parties with the decision (although we thought she was at fault.) after the holidays, the dumped party begged the girl to take him back. He swore he would change- change uni to be with her, change the way he wore his hair...everything took to be with her again. She said no, flatly denying and secretly started dating the guy she left him for. we all thought that was it and it was over. Then, she started meeting her ex in private and having long conversations with him at college. Suddenly, they became inseperable, despite her being with the other guy. They subsequently ditched us and haven't spoken to us since. we tried to support both of them but how have had it thrown back in our faces. They're talking about all of us behind our backs, lying and completely ditching us as friends. We haven't interfered, we've just tried to be good and supportive. what should we do?