What do you suggest if my friend might be mentally unstable what to do?

Talk: Invite your friend for coffee, and explain your concerns about their behavior. If done in a non-judgemental way, your friend may accept what you say, and seek psychiatric evaluation. Equally likely, they may recieve your concerns as a personal attack, and tell you to take a hike. Mental commitment laws vary widely, but someone who is a danger to themselves or others can be committed for evaluation