Find a way to speak to the girl’s parents.

By telling him that you don’t want him stressing over a girl with mental issues, you could be inadvertently teaching him that people with mental or any other illnesses are unlovable.

Help him by opening communication channels with the girl’s parent to find help for her and your son will feel that you care about what he cares about…

That relationship is unhealthy to begin with, but dealing with it in an unhealthy manner will only make things worse.