I can't say that I blame you. Since she put up such a fuss about not talking to him anymore, I'd be unsure of her too. I'm not sure giving her an ultimatum was a good idea, but i don't know what else you could have done. Letting her continue to have this "friendship" wasn't a good idea either. But ultimatums are never really good ideas. I guess sometimes there's no other way to handle things.
I would think that she would value your relationship more so that she would have offered to stay away from him before you had to give her an ultimatum, but I guess that's not so. Perhaps you two would benefit from couples counseling. What do you think? It certainly couldn't hurt. Perhaps you two aren't really communicating, and that's what she's missing. Maybe she and the other guy can communicate better than you and she can -- and that's what she's looking for. There's kind of an "art" to communicating, and being open and honest with one another. if you're willing to go to counseling and she is too, the therapy can help you learn to be open with each other, and even learn how to fight fair! LOL See if she's willing to go. If she's NOT, then you'll know she's not willing to save this relationship like you are.
I wish you the very best. Please keep us informed, ok? God bless and take care. Hugs, Lee