That’s a nice example of several odd things SJWs do:
1. They subscribe to a strong Whorfian position in assuming that using “dumb” to mean “stupid” will affect how people view people who are mute.
2. They don’t notice that no one speaks of mute people as “dumb” any more—the only meaning of “dumb” in common use is “stupid”.
3. They are telling people not to use “dumb” out of respect—yet they reject the notion that they should speak respectfully with people who disagree with their idea of respect.
Oh, and lest anyone object to my use of SJW, the moment those people adopt a name for themselves, I will happily use it, because I do believe people should be called what they want to be called. But until they adopt a name, we have to have some handle to discuss them, and that’s the one the internet has been using for many years now.
Bonus point: Criticizing the friend’s use of “dumb” as ableist is, in SJW terms, derailing.