Asking if it's okay would be nice instead of just doing it, though I would always tell them that it's not okay with me. But don't interrupt someone to ask that question. Wait until she's done whatever she's saying and then ask something like "Sorry, but if I finished a word for you when you are stuck, would that help you at all, or just annoy you?" If you interrupt her while she's stuck to ask, that's embarrassing, and it makes it harder for the sentence to come out if she now has to think about how to respond and finishing her sentence. Also, it's incredibly annoying if someone finishes a word or a sentence and they *get it wrong* so now you have to correct them and continue with a higher stress level instead of just continue as you were. If you can't tell, I'm still in the camp that you should just let people finish, but she may feel differently.