in my experience it never hurts to ask. as you can see by the comments on this post, everyone has their preferences. I don’t mind when someone finishes my sentence as long as they clearly know what I am trying to say. however, I completely understand why people would not want someone to finish their sentence. I always appreciate when my friends ask, especially because they are being kind enough to even ask me how I feel about it. I think your friend will appreciate you asking regardless of what their answer to the question is