I think most people are hitting the nail on the head: don't ask us, ask your friend. It's important for stutterers to work through a block, and not find a different word or let someone finish their sentences. I don't like when people finish mine, unless I ask them to by doing some sort of obvious sign (that isn't mean struggling to say a word). However, I'm a human: sometimes I want people to finish it for me, and sometimes I don't. So yes, check with your friend! In fact, show interest in their stutter and let them know you want to learn more about it.