Make sure you're on each other's friend list.
Make sure you're in the same mode.
One of you might need to log off and back on. If that doesn't work, the other one should try a relog.
If they can see you, but you can't see them, have them send the wing invitation. Then reset the instance by waking out and back in. If that doesn't work, have your friend wake out and in.
Instancing can be a game all of itself.