i can only speak for myself of course but i also feel a lot of relief whenever someone interjects with the word/ phrase i’m stuck on. in some cases, it depends on the manner of interruption (if they’re being nice about it or are acting annoyed) but if you were friendly about it then it was probably okay to complete their sentence. honestly blocking is extremely frustrating & while i can imagine how some stutterers might have wanted to work through the block themselves, i wouldn’t say what you did is some kind of faux pas. at the very least, i’m sure the employee appreciated u not acting weird about their stutter.