I don't mind my friends finishing sentences for me, but it really depends on the situation. It's important not to coddle people with disabilities. Let him finish his thoughts but if you're going to finish his sentence, make sure it's in the spirit of friendship. There are times where I will get stuck on a word for literally north of 15 seconds and its horribly embarrassing; finishing the word at the point doesn't make a whole lot of difference. But, I have to agree with the other people here. You should ask him and see what he thinks. If its done with friendship in mind, there shouldn't be a problem In regards to joking about stuttering - not recommended