Probably the worst thing you can do if you aren’t just trying to stir up trouble is throw around medical terms like psychotic break and bipolar disorder. At that point, you’re basically telling the landlord ‘you should not rent to this person because of a clear medical disability, in violation of the ADA and California’s almost certainly stricter laws on discrimination’ which can really tie his hands. You want to tell the landlord about the behaviors that are dangerous or unpleasant for the landlord and/or other tennants without trying to throw medical diagnosis in. There’s a huge difference between ‘You don’t want to rent to this person because he takes actions X, Y, and X’ and ‘you don’t want to rent to this person because he has a medical condition’ or worse the ‘you don’t want to rent to this person because I said he has a medical condition’.
As a landlord, I can say I would want to be warned of a potentially problematic tenant. Due to my personal views on the subject, I can also say I would take you less seriously if mental health was brought up. Not that I doubt your account, but I’d likely give more weight to the argument “he’s difficult and potentially dangerous” than “he’s bipolar.”
OP wants condo owners to inform this guy’s new potential roommates of his mental issues. That’s out of line.
Again I don’t see any way the disclosure is ‘out of line’ if it sticks to what the person did that OP witnessed. I’d agree if OP was focused on telling other people that OP thinks the reason for the behavior was bipolar disorder or even that the ex-roommate said so. But I took the question as generally should OP say anything. I see no problem (though not absolute requirement either) to say something.
Nor do I think that’s hair splitting. A healthy ‘stigma free’ approach to mental health IMO is to not label people as ‘trouble’ because they have a mental illness. It doesn’t mean you’re sworn to secrecy about bad behavior because you think it might be caused by mental illness or even the bad acting person says so. How do you know what the cause really is, how truthful the person’s statement about their problem is, or for that matter how precisely medical professionals can distinguish ‘ill person’ from ‘complete asshole’? If you stay away from the issue of what causes the behavior (which you don’t know, actually) nothing out of line ethically in telling whoever you want to that a person was impossible to deal with as a roommate, IMO. Again if absolutely forced to choose between ‘you have an obligation not to tell’, and ‘you have an obligation to tell’, I’d go with the second, though may actual answer is just ‘do what you think is appropriate’ (in telling facts of behavior, not presuming to make a diagnosis).