Wife of a stutterer here. Just be normal. Keep your attention on what your coworker is saying and not how they say it. Probably the first few times they stutter a lot it will take some effort to not finish a word for them or look away or be weird about it, but just stay relaxed and don’t be weird and you’ll probably get used to it pretty quickly and soon won’t even notice. My husband’s stutter is pretty severe and I don’t really notice it anymore. ETA: It’s also important to know that your coworker might not stutter less if they feel more comfortable, and that’s okay. If they usually have a lot of avoidances, they may actually stutter more with you if they feel comfortable. (I’ve had to explain to a number of my family members/friends that my husband was stuttering more around them not because they did something to make him nervous or uncomfortable but because he felt more comfortable with them and so was talking more and not trying so much to hide it.) So it’s just good not to focus on their speech but to just be a good listener and a good ally no matter how they are speaking. My husband mostly has blocks, and sometimes if he’s having a very long block in front of someone who doesn’t know him well, they’ll ask if he’s okay, which is really annoying for him because now he’s not just stuck but he also has to reassure this other person that things are fine. And that’s when, if I’m there, my just being like, “He’s totally fine, he just stutters,” in a way that shows it is fine and not a big deal and we just need to be patient can help. But otherwise I don’t really step in with other people unless I know he wants me to. You can just model for other people how to respond, because they may not know. But if you are just calmly, normally listening and responding, then that will show other people how to react.