Conditions that impact specific spoken sounds
There are a certain number of things NOT to do when gaming with a player that has a stutter. These are important. Don’t complete sentences for the person. Instead allow them to come to a completion on their own, at their own time, focusing more on the intent of their words, rather than the manner in which they speak them. Likewise along that route, encouraging the player to speed up or slow down, is counterproductive. Most likely your input will not change the player's ability to speak without a stutter, and may in fact at the end exacerbate it.
From a standpoint of communication and mechanics, communicate with other players the necessity for not talking over each other, and plan for the additional necessary time needed on during player turns.