Honestly, I don't. I think outside forces have played a part in bring on a manic episode, but these have been much more significant things than what someone has said to him. I'm talking about divorce, death, romances, that kind of thing.
It sounds as if you might be wondering if you're to blame for your friend's moods - or maybe someone is trying to make you feel this way. I'm no expert, but I can't imagine that this is the case. As you say, bipolar is an illness - the sufferer is the main victim, but not the only one. On the other hand, I do think that if someone is high, it's easy to engage in arguments with them, and this probably makes the experience worse for them and damages your relationship.
I have learned that it is not productive to try to tell someone what they're doing is wrong while they're in the grip of mania or depression. However, I've also learned that if you spend much time with them it's almost impossible not to do this. It was hard to 'pull back' during the last manic episode, but having done it I am now convinced it was the right thing to do.
I hope I don't sound critical or judgemental. Believe me, if you'd seen how I've acted in the past (and could well do so again) you'd know that I'm in no position to judge. I have every sympathy for the friends and family of bipolar sufferers, and I really hope this gets better for you.