Problem while connecting to a friends world

Dear Mojang Team,

A few weeks ago, some friends and me started playing in my survival world. It was doing very well, but since 1 month, one of my friends can't connect to my world. He's playing on iOS. On his screen appears the message "location server" and in our world we can see him standing in the nether portal, where he logged out. We tried to beat him out of the portal, but we can't touch him. After 1 minute the message appears "He left the game". He can connect in other worlds of me, but not in our survival world. 

Creatings flo