Not all stutters are alike and are at different levels of comfort with their disability. Some don’t want want to acknowledge it, some really want to address the elephant in the room so they can move on. If you are working with this person long term, better just be yourself. That way you don’t have to keep up the pretence all the time. You can have a one on one with them and acknowledge their stutter up front in a comforting manner. You don’t have to necessarily discuss it just saying something on the lines of “i notice you have a stutter, don’t worry, please let me know if I can make things more comfortable for you in way”. Btw it was considerate of you to inquire. I know some people will not appreciate that you felt the need. in my experience non-stutters also get a bit nervous around stutters because most people want to be respectful and not come off as insensitive.