My SIL is deaf and yes I do prep people if they will be meeting her one on one. Just so they are aware and dont act like some idiots do and assume that she’s got a mental impairment because her speech is affected. I do it coming entirely from a kind place as I’m sure the OP did. And you’d think it would make friend B act a bit more sensitively to have that knowledge.
Unfortunately, disclosing that you are/a friend is autistic can have the exact opposite effect - it can cause people to assume mental impairment.
While I don't doubt that disclosure comes from a place of kindness, and I wish that this did lead to people being more sensitive, it doesn't always work like that.
I'm high masking, so people usually don't see me as an autistic person. I'm also highly educated, so people don't assume mental impairment.
However, I do disclose on occasion when I feel it's necessary (if I'll be around people for a long time and don't want them to think I'm upset with them/don't like them if I need to spend breaks by myself rather than socialising constantly, for example).
Most people continue to treat me the same afterwards. Some immediately switch to acting as though I am mentally impaired - talking more slowly, explaining things like jokes or turns of phrase ("you see, 'looking down your nose at someone' doesn't mean literally looking down your nose..."), suddenly my qualifications in a specialist field aren't an impressive achievement/interesting to others - it's a "special interest", as though I'm adorably collecting buttons rather than contributing to academic discourse.
I am aware that it comes from a place of kindness, and I'm grateful that there are kind people like this.
But, as part of wanting to be kind, it's worth being aware that for autistic people that kindness has the potential to backfire, leading to the person you care about having a negative experience they wouldn't have had if their autism hadn't been brought up.