It's advised not to finish their sentences owing to the fact that it will make the impediment worse. If you're patient and have a smile on your face and you help them with the flow of the rhythm and the leftover words, then it might not be a problem. When I used to block or stutter, I never blamed others for interrupting or not letting me finish. It's perfectly okay to do so- just that, your morale should be at a level where such factors don't affect you. Also "Closeness/ Attatchment" is a critical factor that one should take into account before getting into this. If the person having a hard time doesn't mind your support, then it's okay.