Hello and welcome,
Tough situation but it does sound like your friend and his sister have some issues that need to be addressed by the authorities as being attacked physically is against the law.
I can see why this bullying is upsetting both your friend and his sister. By being a good friend to him you will be supporting him and letting him know you care.
If he needs a therapist or physician you may recommend it but he must make the final decision.
I wish you all the best.
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