You sound quite confused. I'm not sure what your question is? You don't want to be with the man your with now, and you don't want to pursue your friend in fear it may ruin the friendship. So you don't want either one?
First, his depression should not be the reason you stay with him since he does not care to seek treatment. This is not your responsibility, it is his. The choices are yours. You are the only one who can decide who you want to be with. If you don't want to pursue the relationship with your friend, then don't. If you do, then do. First focus on issues with you boyfriend. Work things out, figure out if you want to stay or go. You can't juggle two relationships at once and get away with it without hurting someone invloved (maybe even yourself). Once you figure out what you want in your relationship with your boyfriend, then you can focus on the this other guy or not.