Listen to what they say, remember theyre bipolar so be forgiving if they snap at you, stay calm when they're mad, and never try to shut them down during an episode (like by saying, "calm down." or "You're overreacting, it's fine..)
Be supportive and making sure they stay on top of their meds is a must. I am bipolar, was diagnosed nearly 20 years ago. Any advice u need feel free to ask. Good luck. also learn how to spot manic as well as depressive episodes.