Otherwise no one would buy apps, we would all just share them.
Thanks for your replies, so it seems I cant. Not trying to get something for nothing, just wanted to see what's the current set up with Windows apps. When looking at purchasing apps it states "Install this app on up to ten Windows 10 devices." So this is what triggered my question.
She will need to click on her account again and sign out. At this point you can log in with your Store account and download the app