I wouldn't think prefacing people is that bad, especially at this stage when a weird look as can affect them more than feeling they need you to preface for other people, but I would also encourage her to not care what people think and phrase it in a "some people dont know what stuttering is" way as opposed to a "some people are jerks and you should ignore them" way (obviously phrase it better than that lol). Is her main focus eliminating stuttering or being comfortable expressing herself and stuttering with ease? I was clear on your meaning.