You asked, "would you rather that a person that was just diagnosed & is still struggling with his mental illnesses, goes thru enormous emotional stress when I would have to leave bec he wouldn't want me to leave, it would make him extremely sad....."
So if this is your question, here's my answer. Imho I'd leave.

This person needs to learn to handle their own MH issues on their own before involving another. If that person is successful I'd stand on the sidelines & wait.

And no his health does NOT come first. That's martyrdom in my opinion. You apply your own air mask before you help others. That's my opinion.
So if you're ok to stay on the sidelines & be a "cheerleader" so to speak....see how it goes.