Lambikins, there are several "degrees" of bi-polar. when most people think of bi-polar, they think that the manic episodes are such where the person does crazy, harmful things. And yes, there are people that do that.
But there is something called Bi-polar 2 where the manic phase is not exactly harmful, but more what you describe with your roommate. I know this as I have been diagnosed with that. Jane Pauley wrote a book about it because she has it. It is sometimes called hypomania. Just full of energy, feel like you can do 100things at once, but not destructive behavior like full blown mania is. ANd lethargy is a definete symptom of depression.
I would urge your roommate to seek some form of therapy with a good pyschiatric group. I see a Registered Nurse practitioner who specializes in psychiatrics. He regulates my medication. I also see a therapist. Both have helped me overcome a lot of issues and get control over the highs and lows.
Your roommate sounds an awful lot like Cameron Diaz character in "In Her Shoes" the book by Jennifer Warner that was made into a movie of the same name.
Loveable but irresponsible.
There are a lot of people like that. Very charming, have lots of friends, but coast along on everyone's coattails. They are so much fun to be with, but when the fun ends, they are no where to be found. I have recently found out that what I thought was a very close friend is exactly like that. There for the fun, but when the going gets tough, she gets going while the rest of us tough it out.
Even if your roommate won't read it, I highly reccomend you reading Jane Pauley's book. I can't remember the name of it, but it came out last summer. People magazine did a feature on her and so did she on her own show.