Character abandoning the party

I have recently started my first D&D game using 5th edition (I’m the DM). For our first few sessions everything worked fine. The players all had fun and their characters were all developing. During my last session, one of the character’s had a falling out with another (characters not players) so the character left the other two.

During that session I ran the two groups simultaneously (more or less). While the solo character left to explore a nearby forest, the others almost got killed by the villain (one of them did but he was revived).

I would say that this is probably a form of My Guy Syndrome due to the character leaving the party because that is what his character would have done. However, it is getting annoying because, while the larger group is progressing through the story, the solo character isn’t really doing anything.

I can easily get the group back into the same place. My problem:

How can I get the party back together when two of the characters dislike each other?