This is so strange, and I have questions (just putting them out there, I know we don't have the answers yet).
What was the argument about? It must have been pretty bad for him to leave and go on this long of a walk. It's one thing to cool off by walking around the block. It sounds like he had no intention of coming back that night with the path he took and also phoning people for a ride (unless he stormed off and didn't realize how far away he actually got and was hoping for a ride BACK to in-laws' place?)
Why did friends not come pick him up? (NOT blaming them here at all, just curious what the circumstances were and what exactly was said over the phone). Or, I guess, DID any of them try and pick him up?
I am not getting a good vibe from this, unless a friend (or someone else) DID pick him up and are allowing him to lie low for awhile. Otherwise, how would he be surviving this long without buying stuff? I guess he could have cash on him, but you'd think someone would have recognized him somewhere if he went to a store. Maybe not. How would he get very far? If he hitched a ride, I would hope that person would come forward to LE unless he convinced them not to.
An adult can choose to walk away from their life for a while or for good. I just hope if this is what happened, he will at least call LE to say "I am alive and I am fine and chose to leave" so his family isn't worried about what happened to him. This would be the best case scenario right now MOO.