Tried this several different ways.
I had a friend try to enter the cave in my world before and after me, but each time he would get an error and have to login again. This was also like this in the beta, but after an update I thought it was fixed.
Also I tried going in caves alone and then inviting him, but the invite button disappears.
This is bad lol I need help with the ruins >.<
Try having someone enter a private hosts cave.