If you've been with a girl for a few years and you may even live together,
What do you "may" live together bro? Either you live together or you don't.
and she did something like violate a boundary eg hanging 1 on 1 with a male "friend" (behind your back..she doesn't know that I know) and being disrespectful..
Would you tell her why you're dumping her or just say it isn't working out and give her no reasons.
Bro there are so many unknowns in your question.
How did you know she hung out with him? Did you follow her like a detective and wait for her to come out of a restaurant and take pictures? If you are doing that type of thing it's good to know your relationship is based on trust and honesty.
How do you know they didn't talk about your birthday party?
How do you know they slept together?
How do you know she is not trying see how jealous you are?
It's unlikely but before you start ripping her to shreds and telling her to go to hell, find out the whole story. You are living together so you obviously trusted her on some level.