How to help someone with manic depression, who doesn't necessarily want to be treated?

Agree w Dr. Ali. It is hard to help someone who is unwilling to accept help. Call 911 if the person becomes a risk to self or others. Work to maintain support while simultaneously keeping healthy boundaries.
They have to want it. Your friend has to want therapy and medication, and all you can really do is encourage him/her to seek both and engage deeply and meaningfully in the therapeutic process. You simply cannot do it for him/her, much as you'd like to. You can either be his/her friend or therapist. You cannot be both, and even though you want to see him/her get better, trying to be both simply will not work.
Bipolar. Support, guidance, educate, reassure, role-model. Finally, acceptance of what you cannot change.