I'm sorry your significant other's feeling this way. Personally, I normally feel like wanting to end the world when I'm depressed and hopeless. But I have read about other people's accounts that during mixed episodes or even during what seems like stable moods, a person can still become suicidal. And it's because they know what's ahead... they know that the depression or a manic episode is not too far from sight so they want to end things when it's still "good". I've felt a bit of this, where it's the anxiousness of the next coming episode can make me not want to live through it. I hope I'm making sense O_O

For the "what to do" part, the only thing I can think of is contact a trusted doctor or therapist. He's in a good place to have you by his side, but you don't have enough eyes to keep watch on him and make sure he doesn't do anything. It's good he seeks your help though. Well, I hope this helps somehow, and I hope things turn around for the better soon!