Best way to deal with someone with mental health issues?

Long story short. We have a very on off history. We've been intimate but it's never gone all the way. Recently, she has developed psychosis and her depression has gotten worse. She is seeing someone so I have withdrawn. She called me up recently saying she was thinking about me and how overwhelmed she feels.

I feel like I should at least be supportive because I do care about her and this is serious stuff. On the other hand, if she is seeing someone then isn't this his issue?

Best way to deal with someone with mental health issues?
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