Many of the mental illnesses do not make it impossible to get higher education, they only create problems in contact with people, and if they are diagnosed, then the owner of one of them has even more chances to enroll, because the university will provide him with an educational privilege. By the way, a friend of mine with a split personality is studying with me at the same university.
Of course. According to my observations, at least 30% of students of psychological faculties can be characterized as "having some kind of mental deviation." And this is logical, young people strive first of all to solve their problems. However, not many succeed in this.
If he / she has no problems with learning, he is socialized (does not walk naked, does not yell about snakes, etc.) and is not dangerous, then why not. It all depends on the nature and degree of deviation. And some specialties may have restrictions (pilots, state secrets, etc.)