The thing that struck me in your first post is that she wanted you "to stop" and help her. You need to tell her that as soon as you finish yours, you will help with hers. You don't need to complete the entire project before you help, but you need to complete that step or come to a good spot where YOU can pause. It's one of those things where if you guys have been encouraging this behavior for a while, you will need to be very firm.