The main issue I have is that he is becoming worse, not better and I don't see any end in sight as he escalates his cynical conversations and rants.
If things are getting worse, more frequent, then that's a concern. Admitting mild, but it doesn't sound mild (whatever is going on, which is likely more than one thing, even if one thing is driving the other things.)
Bipolar is complicated, even to diagnose much less treat effectively. The symptoms can be classic or subtle, some don't get low, some get angry on the upside others get really happy, some don't get very high at all. The key thing to keep in mind that the upswings, when big, feel really, really good. Empowering, aggrandizing, full of energy and ideas. It can be as addictive as any drug you can think of.
But I do have a suggestion that might help nudge your friend in a good direction.
B vitamins are helpful in mood regulation. Suggest he gets a multivitamin with lots of B complex, like Men's One a Day. Take one every day, see if that helps.
It probably won't help, but it can't hurt.
But what it will do is give him a way to focus on his issues. And let him know that if he skips days, that's a sign of a swing happening. That way he has a marker that will let him know early when a swing is happening. He will be thinking about it every day when he takes his pill. If he starts to see a pattern of skipping pills and more problematic behavior, that will also focus him on the issue more.
That might lead to more of a willingness to seek professional help, tracking the symptoms, being aware of the shifts. It might also lead to more focus on what goes on in interactions. "I skipped a couple days, I am starting to spiral up, I need to keep that in mind when I get excited about something talking with people."
For conversations, I suggest you use this as your touchstone: "Is it interfering in your life?" So, it sounds like that interferes with your job/friendship/relationships. Do you think that gets in the way of getting more out of work/family/friendships?
Assuming your friend wants better out of life, this framing can also help lead to more self reflection. Key is to frame it as the condition interfering with life, not the person/personality.
You are a good person to try to help, but as you have found, it is very hard to endure this kind of thing. Not one time, but over time the stress builds up. So if it comes to it, and you need to cut ties for your own mental health, you should say that. But also say that if he gets help, you will be ready to be back in his life. If you have to have that conversation, link it to previous talk about his condition(s) "interfering in your life".
Good luck. You'll both need it.
