One thing that's good to keep in mind is that stuttering is always worse for the person who stutters than those around. People will respond to me empathetically, saying they don't mind, but that's always the least of my troubles; fact is, stuttering sucks and is endlessly frustrating /for the person who stutters/. Best you can do is ask what their preference is, if they want their words finished, and what you should do if they can't continue their sentence.