You are a good friend. What you are already doing is a good example of that. Many people think they are helping when finishing sentences, and I honestly can't blame them for doing so. For the most part the intent is good and we know it. You would help a friend carry heavy bags of groceries wouldn't you - so why wouldn't you help the same friend with something else he/she is struggling with? I can of course only speak for myself, but when I'm around people that I KNOW don't give a hoot about my stuttering and isn't visibly bothered by it, I stutter less and less in front of that person. I get more comfortable if people also ask questions about it and seem genuinely interested. Just avoid "helpful advice" such as "Don't worry about it!" or "try to relax before you speak"... yeah. Like that hasn't occurred to any of us.