You don't even need to have someone else running their character. Just say that the character is busy/drunk/sick whatever match their role-play. Or simply have an agreement with the PC that despite being unrealistic for the sake of the game, nobody will wonder why a character can be absent in the middle of space or a dungeon without any justification.

I understand that in game with a big focus on party balance/tactic it may-be different but this kind of the game is the exception not hte norm