What can be done for a friend with manic depression?

Support : Support your friend to get appropriate medical help and counseling. Don't judge them just help them.

Sensitivity: The best thing you can do is guide him /her to accept psychiatric help. He will need both psychotherapy and medical treatment. Unfortunately one of the most common patterns with manic depression is avoidance of the diagnosis and of treatment, as perhaps you have already seen. Therefore great patience, kindness and sensitivity is needed to guide a person toward the help he or she needs.

He has to want it: Your friend has to want therapy and medication, and all you can really do is encourage him to seek both and engage deeply and meaningfully in the therapeutic process. You simply cannot do it for him, much as you'd like to. You can either be his friend or his therapist. You cannot be both, and even though you want to see him get better, trying to be both simply will not work.