So, we both play using internet (xbox friends, not LAN). She had just bought minecraft when she couldn't join my worlds and wasn't able to see them in the friends tab, but I could join her's. When I hit "Invite player" it shows that she is online and I can invite her, but she doesn't receive the invite. But now I can't join her worlds either.

And yes I checked the multiplayer tab settings of the worlds. Yes I can join other servers.