I mean, ultimately it depends on the person and I’m not trying to speak for everyone here, but in my personal experience I didn’t like when it happened. I felt like I couldn’t express myself properly, that I needed always somebody by my side to help me, to rescue me, and that made me feel miserable for a really long time. Nowadays, for me it’s pretty rare and it has been a long time since I haven’t been able to finish a sentence. Acknowledging that, many people (like me) have trouble under pressure, for example in our schools, unis, jobs, etc; I think one of the keys for fluency is confidence, but when somebody is always finishing your sentences, you kinda become reliant on others. This means that next time you’ll be expecting your friends or family to finish your sentences, and that generates a comfort zone. “Why should I talk if X or Y can say it for me?”, that’s what ran through my mind everyday when I was younger. Now that I live alone, I have to do everything myself, even phone calls that were my nemesis when I was a teenager, I have to make them out in a language that is not my native one. I even have to speak for some people that can’t speak English in a fluent way (I’m not saying I have the best English, I still have to learn a lot). But ultimately, it helped me a lot just to face many situations by myself. This is not an absolute truth. Everyone is different, so we don’t know what’s going on in everyone else’s mind. We have different backgrounds, different ways of being, of expressing; there’s not a magical solution for this. The only thing that I suggest you is: be patient and make sure that you make him feel heard, that he has a voice.