It sounds like you are both in your own Teams org's. If she invited you to her org, which it sounds like since you show as guest, you must switch to that org, on the mobile app, when you click your top left menu, at the bottom you should see her org there to switch to, if you don't then you haven't accepted the invite to her org so it gets added to your listing. She may have to remove, and re add you again, but it can be very problematic if you didn't get the initial invite and might be a problem you won't be able to solve. If you are just doing chat might not be the best product.
