In the case of your friend if the disc is unreadable they can’t play Forza Horizon even if cleaned and there is nothing that can be done about it I’m afraid.
In your case with your temporarily disabled message, you can’t discuss bans in the Forza Motorsport forums (as the temporarily disabled message is a ban) and so you would need to email forzafb@microsoft.com with your gamertags you use on Xbox Live and description of the ban you want to discuss with T10 in your email (you may not receive an individual response, but will receive an automated response).