I think it depends on the way its done. Usually when it's done in a casual way (like not when someone is doing a presentation or during a debate or Smth) people don't mind (from my opinion as a stutter). HOWEVER if you're talking about how you think he might have felt then that depends on how you do it. The way you did it seems casual and like you're catching up to his explanation like you would when talking to someone in general. The way people might not like, and what actually would get some offended is if you say somehting like "are you trying to say SIDE DOOR" that may come off as a bit offensive. Generally some people like it and some don't. But as someone who doesn't always like it when people complete my sentences, the way they do it can COMPLETELY change how I would feel about it or wether or not ill be annoyed by it