I have forgotten a great deal of things, when my episodes are really bad, but I still obtain a general sense of what had happened. Like 'same routine week' or I'll get an insight, if I think hard enough, of a piece of conversation. Otherwise, it's just a blank sense of time having passed - not wiped clean, just kind of, tucked away and hard to recall.
I've not heard of a bipolar having changes like you've mentioned, not even in drastic switches between mood. If the behaviour had continued and, say, other people had witness his change of tone or forcefulness, then yes, maybe.
Please keep in mind, it may be something else entirely; he may have seen a physician and been told bipolar, but it may be a different disorder entirely, when these new symptoms are taken into thought.
Or, and not judging here, he may just be saying he's bipolar [some people do, in order to get away with things, which is weird]. And/or he may just be lying when he says he doesn't remember.
If he seems genuinely distressed, encourage him to either see his doc or find one, just to be sure. Also help him to find out whether this has been a one-off occasion, or whether others have noticed it when talking with him, and just not mentioned it.
Never heard of this happening with anyone. I think someone is lying, either her or him.