As someone whose brother is mentally ill, my deepest condolences to your friend. It is not easy. The most important thing to understand is that they are not like us. It is unreasonable to expect them to function at the same capacity as us. This doesn't mean that we treat them any less than us, but it also doesn't mean that their poor behavior (especially aggression) can be excused. There is no black and white formula for this, and discretion must always be thoughtfully exercised within the context of your friend's family.

Speaking to your point on Poly, depending on the severity of your friend's brother's condition, it might actually be unwise for him to continue with school. You can't expect everyone out there to be a saint and treat your friend's brother with empathy and compassion. The world doesn't work like that, and he will be beaten down if the environment is not inclusive. If he can't cope with it, all this will do is serve to exacerbate his condition. Instead, you could ask him to look into social enterprises that he can work at, which provide a much less stressful environment designed to accommodate special needs personnel.

There is honestly way too much to talk about, and every situation is so unique that I can't adequately contain everything in a single comment. If you or your friend would like to talk, feel free to shoot me a pm. All the best!