There's a few threads on here about this is you can find it in search. Fitz is right, though some people are super sensitive about it and don't want to talk about it. There are many reasons why we usually don't like it: It's impolite to finish someone else's sentence; it makes us feel rushed, like we're taking too long and we need to hurry up; makes us feel inadequate and failing to act up to *acceptable* speaking standards; makes us more self conscious and highlights stuttering as "bad" and a "problem"; sometimes you guess wrong and we have to take more saying no and restarting the process of trying to say the word, etc... Don't let that long list make it sound more dramatic than it really is. We just want to be treated like anyone else, we just require a more patience from the listener. If the co-worker is really stuck a struggling on a word and you know it, you can use your judgment on whether to finish it or not. Even if you guess wrong, it's not a big deal. The main thing is you don't finish every word because you're impatient with his stuttering, but that doesn't seem to be the case. Thanks for asking!