God, I'd love to say "well, this, then that and that and everything will be OK."

You probably know that I can't. I can only say to you that when I get into a 'high'(I'm bipolar II, so I only get hypomanic, which is mildly manic), nobody helps with saying "yeah, that might seem a good idea to you but hey, you're hypomanic, so it really isn't.". I take mood stablilisors(seroquel), and that works kinda fast for me. Maybe it would be possible for somebody to tell her something like "hey, if you're not manic, this won't change a thing, so just take these mood stabilisors for like 3 weeks. Just to let ús be sure that you're not having a manic episode.".

Like you said, it would be a good idea to leave that conversation too somebody she totally trusts nów. Not somebody she loved her whole life but had a quarrel with just the other day. I tend to trust the people that are supportive of my ideas during that hypomanic episode more than somebody who started the conversation with "what the %#@&#! have you been doing now? You're manic again, aren't you?". When in a manic state, you tend to have feelings about people that are based on the last couple of days instead of the last ten years.

But above all, I wish you a lot of strength during this hard period. Just know that when she'll take lithium/mood stabilisors, she will be like you used to know her again, and I think she'll even be less depressed.