Unable to join game session. Players not on latest game version. Happened in Aug

Since you own the game on your Xbox, the problem is on your friends end. He game shares with you to be able to play the game. The only fix I've found rn is that he would have to Uninstall the game and buy it himself on his Xbox. The other solution is if he signs in to your account then switches to his having both accounts signed in, but if you try to sign in at the same time it will kick him off.