I have personal experience with dealing with several people afflicted by mild manic-depression. It's not easy.
What I write is from my limited understanding of the disease, and my personal experience with people afflicted by it. Your mileage may vary.
Depression seems easier to deal with - you just need to be there for the person. Let them talk. Encourage them to keep going. If you're supporting the person, don't ever give up on them. By doing whatever you can, and making sure the person knows you'll be there, and by feeling completely impotent, you help.
When manic, a person loses much of their judgement. They may become promiscuous, violent, or anything. It's impossible to tell what will set them off when already manic, or even what will incite mania (some manic depressives can cycle in minutes, some cycle over years, others are anywhere in between).
I've had to spend hours talking to someone, manic at the time, completely irrational and running on emotion. I had to pry at this person's emotions, carefully phrase my words, acknowledge feelings, prevent the conversation from going on tangents, and just about become completely exhausted, to convince them that a decision would work out for the worse.
Once set off, the common description is one's "motor is running". Once the motor starts, it seems to feed itself, revving faster and faster, pushing the mania faster and faster, further and further, as though trying to break, so that the individual can rest. Mania can last from seconds to weeks.
When manic, people will generally start many projects and never finish them. For example, one might wake up at 1:00 a.m., and start painting a room. At 2:00 (with open paint cans, and one half painted wall), they might decide to make breakfast. At 2:15, they may decide to wash the car. At 2:25, vacuum. At 2:50, watch a movie.
If you meet someone who is manic depressive, the first thing to realize (and this sounds rather harsh) is that it's not your problem. It is their problem - you're just a witness to it, and you may become support.
The next thing to do is decide if you want to support this person. Remember, that means you need an unbreakable bond (even if the person tries to seriously injure you, or themselves, you can't give up), and that it will be a lifetime commitment. If they're not family, and not a close friend, and they're not your patient, then stay away. The person is a danger to themselves and possibly to those around them, so you need to be sure you're willing to get into a really bad spot, for a very long time (years) to help get this person out - if they can be helped at all.
Third, you cannot be the only support for this person. Other family members, friends, etc.
must help. No matter how strong you are, you're not strong enough. It may take years, but you will become worn out. Other people need to be around so that you can take breaks and recharge. And you need to be around so they can take breaks and recharge.
Fourth, the person needs ongoing medical assistance from a qualified psychiatrist (a psychologist can't prescribe medications). There are many medications that can help, but people may develop a tolerance to them, making them ineffective. Also, not all medications work with all people, so some trial and error is necessary (which can be extremely discouraging for the sick person - but you can't give up! You
must remain optimistic that a cure is out there!). Also, the medication may help, but it may not be sufficient to solve the problem.
Finally, I recommend reading
Call Me Anna. It made it a little easier for me to understand what was going on.