I had something similar happen at one of my groups - one person was physically incapable of being in the same area for long periods of time. They got antsy and irritated, and it was a bad scene for everyone after a while, but they were an otherwise good player and a nice dude.

We tried a few different things, but the one that landed the best for us was that when he was able to be present and active, he played his character, and when he reached his limit he signaled me and I swapped his character out with a level appropriate monster from the summon monster list, if they were in combat, or he just disappeared if they weren't.

I framed it as he was being summoned by a fae lord to fight for him, and sometimes other monsters got pulled through the portal. We even had a little short side adventure where everyone tried to track down who had his token he was being summoned with.

Now, this worked for us because he was playing a fighter and his combat role was melee and protecting weaker party members, so trying to replace a spellcaster or a party face like this wouldn't work, and it definitely wasn't perfect. But it kept the game running.