Yeah, that is the hard part- convincing them to get treatment and stick with it. The person in my life that is Bipolar started taking this drug, Lamictal, and within a couple months they were a completely different person and have remained so for a few years now. A couple of weeks ago, they mentioned to me that they thought they might not really be Bipolar and were thinking about going off the drugs. My heart just sunk, but that is *very* typical with Bipolar.
First and foremost, if you haven't done so already, I would sit down alone with the person (the OP or anyone else dealing with this) and just try to have a one on one heart-to-heart conversation with them. Just showing you care and that you legitimize their illness can make all the difference in the world. If they feel like you don't understand or look down on them, that will definitely affect how things go as well and not in a good way.
At any rate, you can't force a person to stay on their meds and it can become quite frustrating. You can only try to reason with them and progressively enlist the help of others if you can't make headway with them. If that still doesn't work, then I would try to negotiate with them, ie. stay on your meds and get treatment or you may have to move out. Every situation is different, but I would start with that.