That Cartoon Junky
What to do as a Gm: When a PC splits the party.

There are two options to choose.

Option one, contains many steps. Talk to the PC in question who split the party. Ask him why he did it or his character (for that matter) deiced on this course of action. If the Pc has no reasons for this and did it on purpose, my best advice is to kick him out of the group. Now if his character did it and he seems to have good reasons. Ask the person, how you as a Gm can bring the character back and also have a stronger bond to the party and story. You can bring back the character in game through an additional game with just you and the PC or write time to where the party split never happens. Writing time is never a good idea, and just be used as a last option. Hopefully this action will not occur again if it does proceed to option two.

Option two:

KILL THE CHARACTER AND MAKE HIM AN EXAMPLE FOR THE REST