My brother is a special needs adult. As it was the two of us, it was very hard for me to relate to my brother past a certain age. It also was very embarrassing the questions I would get from my friends as to why he was still living in my parents house, had no real job, and no girlfriend. My parents did all they could to help my brother. He had and still has many social and learning issues. In our community it is a very big stigma to have a sibling with a special need with shidduchim. While I did not meet my husband through a shadchan I have no doubt it would have been an issue as to what disability my brother has and if it would be passed on to any children that I would have.
I also understand what it is like to have an older sibling that you have surpassed. In some ways, I feel like I never had an older brother. It is very hard and psychologically draining to be the older sibling in some ways when the person is older than you are. I have no doubt that the younger siblings resent their older one in some ways, or at least are as tolerant of their sibling as the parents are. The parents most probably have their hands full with the other children that they are now unable to give the time and energy that they may have once given.
Your friend should defiantly talk to her siblings about how she feels her brother is being treated. They need to learn to respect their brother as a person. It will not be easy. The parent should also seek counseling to help the family embrace their son as part of a the family and not as a “taboo” which needs to be swept under the rug. From experience, while my parents did the best that they could, they did the best that they knew how. My parents were sometimes harsh and unnecessarily critical of my brother which was in my belief more damaging than any condition. If your friends family does not fix any negative behaviors or attitudes towards the brother, it will effect him negatively for the rest of his life, and theirs as well.