At the risk of stating the obvious, I believe jobs which are not time-critical would be best. For example, a production type of job with flexible hours where the person's productivity, rather than hours coming and going, are the basis of pay. I guess the short answer is "work on commission". Even then, some employers will want steady hours, so you have to work hard to find the ideal situation. I was lucky enough to find jobs like this when I was in college; I had a few years experience in electronics repair and I found jobs doing repair work on commission. I worked the hours I could fit around my schedule and my bipolar ups and downs. I didn't know I was bipolar then, and most people, including family (and, I suppose, me also, to some extent), just thought I was unreliable and lazy. So it is important to find ways to communicate your situation to prospective employers, even if you cannot just say "I am biploar and can't work regular hours". In my experience, there still are not many people who know what bipolar is, and fewer still who are "OK with it". I have had my own businesses and I hired biplolar people when the job allowed the flexibility they needed, but I did not know very many other business owners who were as understanding as I was.
Another possibility is to work on jobs over the Internet. There really are such jobs available, but, there are also a lot of scams and dead-ends advertised on the internet, so you have to be careful. The rule of thumb is "if it sounds too good to be true, it probably is". For example, an ad which claims "make thousands of dollars a month with no training and only working 10 hours per week" is probably "too good to be true". The more "down to earth" the offer seems, the better the chance it is valid. Very few companies hire people to type or stuff envelopes etc., for high wages or commissions. An internet job or other opportunity will require you to have a high degree of initiative and self-motivation. I was able to succeed because, once I managed to get up and get going in the day, I could work hard and could work until late at night. I tried modifying that pattern to fit the college time outline but I did not succeed. When I accepted my limitations and made sure I took classes which were not so early I would probably not make them most of the time, and jobs which were either flexible or were "swing shift" and even "graveyard", I was able to get through school with an engineering degree. I finally started my own businesses so I could set my own hours and not have to wrangle with things all the time.
I hope there may be something useful here. I just wrote everything I could think of. It is far easier to give this advice than to follow it, however, since your friend has your help, he has a great advantage over my situation, as I did not have anyone doing what you are doing. I commend you for it. I also did not know I was bipolar, as that diagnosis was not very common when I went to college. Mostly, I was often humiliated, yelled at, and so forth, for being "lazy", "immature", "afraid of work", etc. I hope your friend does hot have that to deal with, but if so, try to offset those awful experiences with as much encouragement and positive attitude as you can get away with.
The last comment I have may be the most important. DON'T GIVE UP ON YOUR FRIEND! If my wife was not absolutely the most understanding, loving, supportive woman ever born, I would have failed many times. To have someone like you supporting him, your friend has a priceless ally to help him get through school. I hope he is majoring in something which will offer the flexibility he will need later on, as he deals with bipolar problems. You just have to keep trying everything you can think of, and, if possible, learn to push failure aside and not let it wear you down, as bipolar illness will invite failures; you have to adopt the attitude that those failures are really opportunities to learn more about what you cannot do so you can focus on other things you may be able to do.
Good luck. Sorry I can't be of more help. Your friend is lucky to have your support!