If you know someone with bipolar disorder and they're having a particularly difficult time, don't avoid the subject. Ask them straight out if they're thinking of hurting themselves. Talk about it. Ask them if they're thinking about killing themselves. As them if they have a plan. If you have any suspicions, don't leave them alone. Talking about it will not make it more likely that they will act on their thoughts. That's a myth.
When someone with bipolar disorder becomes manic they can act recklessly and take risks. They can also become psychotic. As well, when they are depressed, the hopelessness and helplessness can become too much to bear. Perhaps they're feeling trapped. When in doubt call 911. They need a mental health professional. Always take talk about suicide seriously.
Prevention of suicide starts with awareness, conversation and medical treatment. Don't be afraid to talk about it. You might just save a life.