This article just came out:
Does CBD Really Do Anything? (TLDR: Yes for epilepsy, the jury is still out about almost everything else.)
So, I'm no psychiatrist, or psychologist, or even a first year psych major, but I happen to know a lot about bipolar. What you describe does not sound like bipolar to me, because pot is
not a replacement for Lithium. It might help some, but it's far from a cure (subjectively I think it does help, but I haven't seen any research published that answers the question one way or the other.)
I'm not saying that anything you said is false, just that maybe whoever diagnosed her got it wrong (psychiatrists don't make accurate diagnoses 100% of the time). Maybe she was taking drugs she didn't need as a result and then weed cured the real issue that the doctors failed to diagnose, allowing her to ditch the drugs and their side effects (which weren't really solving her problem anyway because she wasn't really bipolar).
Don't get me wrong, I <3 weed; all I'm saying is, it doesn't cure bipolar, and it's not exactly super rare for psychiatrists to misdiagnose someone. But it's great that whatever her issue is your girlfriend found a solution for it in pot.