It’s not normal behavior. Hypomania and mania CAN influence your behavior, but everyone is responsible for their own actions. I have NEVER cheated on anyone before. I would NEVER do it, even if I were extremely manic.

The fact she denied it and then tried to cover it up with an excuse (“oh, he wanted to, but we didn’t!!”) means she knew she was wrong but did it anyway. I’m not saying you should dump her or not. You have to come to that decision yourself. But hypomania and mania are NO EXCUSE for doing that.