Okay, so there are quite a few people here who are very confident in saying that you should never finish sentences for others, that it's extremely disrespectful, 100% certain, etc. My insight, and of course, I stutter, is that it can be okay. Actually, it depends on the person who stutters, so it's hard to give you an answer :) Perhaps, generally, it's better not to finish sentences for others, but hey, there are plenty of stutterers who don't mind and even find it helpful! I hope this adds some nuance.