I never finish a stutterer's sentences. I've only known one though, and I was never interested in what he said anyway. I always just pretended to listen to what he said, and interjected my silence with stuff like: yeah, huh, really?, oh, etc, said in a way that would indicate I didn't give a crap. Wasn't because of his stutter though, he was just the kind of person who would start flinging tonnes of boring stuff at you. Like if he saw me playing some game he'd come over and start spouting a lot of crap I didn't care about that is only remotely relevant to the game. Stuttering didn't make it better though, only meant it would take longer for him to shut up and let me enjoy whatever I was doing.