Refuse to slap a verse on it. We can minimize someone’s struggle by simply quoting Scripture. For example, a person struggling with severe anxiety disorder isn’t helped by sending them a text that reads, “Be anxious for nothing.” That’s the equivalent of reminding a prodigal’s parents, “The Bible says, ‘Train up a child in the way they should go; when they are old they won’t depart from it.’” When someone is either manic or severely depressed, they probably won’t receive what you’re saying the way you think.
Don’t assume it’s a “sin” problem. Mania and depression are serious psychological issues. It's true that sin has worked its way into the issue, either as a trigger or in their current behavior, but mental illness is too complex a problem to simply lump into the "you're in sin" bucket.
Don’t confuse mania with demonic possession. Severely manic persons can exhibit wildly, strange behavior but they are symptoms of a deep psychological diagnosis.
Don’t distance yourself from them.
Educate yourself about their illness.
Set boundaries. Mania episodes can bring unhealthy behavior.
Encourage them to get professional help. It’s more than the church can handle.
Create space for their mental illness.
Care for the caregiver.
Visit them if they are hospitalized.

Grace for the Afflicted: A Clinical and Biblical Perspective on Mental Illness