the hardest part may be not letting him back
Because both he and his girlfriend are not worth the trouble of maintaining a close relationship with. It seems strange that neither of them understands that their behavior is exhibiting classic signs that they only care about their own feelings. It is something that monogamists have a hard time understanding because in their eyes, they believe they care about their own feelings and their partners. To be honest, without the introduction of a poly-like relationship they would likely never know that they're showing tendencies to only be worried about their own feelings. Most people who truly are incapable of being concerned with how their behavior affects others become extremely adept at manipulation and so maintaining emotional control may keep them in denial of the truth, that deep down they are only concerned with how they feel, but not their spouse.
What they did was wrong. I am sorry you have to go through this.
If she was capable of genuinely caring about others' feelings, she would know that she should not offer him or you the choice to choose. She should bow out, out of respect for both him and you if she honestly cannot handle non-monogamy. The way she went about it is a sign that she likely does not care about his emotional health.
I normally refrain from giving advice like this where I attempt to get into the minds of others because I believe it is unfair and you should not be taking advice from people online in regards to your relationship and what you should do. It is impossible to present the whole story, as it would be a novel.
But I do feel like you should know that a person who genuinely cares about how their behavior affects others (more than only caring about others when it directly affects them) they do not treat other human beings the way they treated you. If she honestly cared about the boy that left you, she would not have forced him to choose between her and you..
She may not be uncaring, but if she isn't then she is just plain immature and/or ignorant. And it is too bad your partner was fooled by her manipulation or ignorance because chances are she waited to give the ultimatum until she was confident that he could be wrapped up enough to choose her.
When someone uses that criteria to give the ultimatum (waits until they believe issuing ultimatum will result in them getting what they want) it is better described as manipulative controlling behavior. It has less to do with her being tormented by whatever the occurrence she is putting her foot down against, and more to do with her being out to get the most out of others regardless of how it affects the people she is taking from.
Yes, the excitement from new relationships can blind people. But he should know this, and while it can be an acceptable mistake to make once, I would be not allow him back into your life the second time. You deserve close relationships with people who genuinely care about your feelings when you are a responsible friend. If you are responsible with your honest feelings and behave as one does who genuinely cares about other's feelings, you deserve close relationships and intimate friendships with those whom recognize what you have to offer.
I do not believe that he and she truly know themselves, and people like that require an angel-like or sainthood levels of consciousness if you are trying include them in your life completely without experiencing pain that may break your spirit. Those who only have an ordinary understanding of LOVE, are very painful people to teach a more complete understanding.
Please take my words with a grain of salt, because only you can be the best guide in your life, but being the best guide to yourself requires a true understanding. I could read a thousand page volume on just the last ten years of your life and still not have a adequate understanding. Not to the point where you should take my advice. So please do what you know is right, and not what I think may be the right thing to do