Hi Kathy,
welcome to neurotalk!
There is much information about mood disorders here and at psych central our mother forum. There is a sponsored resource link above "what is bipolar depression?" You are being a good friend in trying to get information....having a good support system in place really helps in any relationship, learning about mental health issues takes time and patience.
Decreasing stressors in your life also helps.
Eating well and trying to have good sleep hygiene helps most people.
There are alot of really nice people out there undiagnosed living normal lives. There are alot of people who suffer with anxiety, panic attacks, post traumatic stress disorder, OCD, ADD, borderline personality, substance abuse users, sleep disorders, depression and people who never seek treatment and could benefit from medications and talk therapy.
Depending upon your relationship with this friend and how it is effecting you, you get to decide how to broach the subject.This is hard for an outsider to tell you what to do. HOw do you bring up a delicate topic in any relationship? gently!