Disconnection issues

hi all, so recently my friend has been disconnecting from halo 5 matches about 75% of the time. He didn’t used to and we were always able to play, however before we would have to quit halo and keep restarting it until we could eventually join eachother, as annoying as that was we were still always able to play. But now he is able to join me but he lags out of the game almost all the time, every other game is fine and his internet is fine but he lags out of halo 5 games and it says he has lost connection to the servers. Does anyone have a solution or know of where I can find a solution? Would like to get playing with him again as we pretty much have played together since kids. Thanks in advance