In my experience, and in interactions with many other people who stutter, it varies. The majority of people appreciate being able to finish a phrase without the assist. Your interaction seemed pretty benign, maybe he appreciated it and moved on. It’s an accumulation of that occurring several times a day for a number of years that can chip away at ones worth. An alternate would be to wait for him to finish, you both feel some bit of relief and move on from there. They’d likely feel not only relief but a small victory by not avoiding that moment themselves too.