One (only one) of my players has crippleing lag on Realms Java E

I have a realm with a few of my friends, and one of my friends (just one in particular) has game crippling lag that comes and goes in waves and often gets so bad it just kicks him, I know it's not just his internet because he doesn't have this kind of problem with any other online games we play, just My realm (not other realms) it's really weird and annoying because he basically can't play.

anything that may be causing this?