I don't know how contracts work in that sense, but if he is claiming mental health issues, they should be in the know about what he is doing to get better if he isn't going to use their doctors.
I don't think HIPAA plays into this.
Seems like it would fall under similar category as an employee being required to have a doctors note to return to work when they've been out for more than 3 days. The employer doesn't get or need to know why the employee received medical care, just that they received it, are fit to return to work and what capacity they are fit to return in.