Yeah, sometimes it may be necessary for you as an outsider to intervene. This is a whole lot easier if you are a close family member or friend. What are his thoughts on his changing behaviour? Have you spoken to him about it? I have a friend at work who I've confided in, and asked her to intervene if necessary, given her full right to take me aside if she thinks I'm behaving out of character or even push me to go to hospital if needed.
Especially if you are going through a manic episode, you cannot discern right from wrong, and you definitely don't realise you are doing anything wrong or terribly different.
Thanks for the support you have shown your friend!