Here's the only thing that makes sense to me (rampant speculation.) Mom and TC drive to 7/11. TC jumps out of the car and pretty much says don't wait for me, I'm just going to walk home. With his condition, that probably raises her suspicion/concern a bit. Then again, he is 22 and doesn't seem like he wanted to talk to her about it and probably didn't wait around. She pulls out of the parking lot to make him think she's leaving, but she has no intention of doing that. She pulls in up the road to turn around and wait. A white car passes her driven by a guy with jet black hair. She sees it pull into the parking lot, which perhaps is not entirely in her view from her location. She pulls forward and catches the quickest glance of a guy with similar build to her son jumping in the car and shutting the door. They drive off and she's trying to decide - was that my son or not? Should I follow him or not? Again, this isn't a teen, he's a 22 year old man. So perhaps against her better judgement she decides that he's in his friend's car, so she is no longer concerned about him walking, and maybe the friend is really what he needs right now if he won't confide in her. She goes home and waits for him to come back. Meanwhile, her son has been milling about the store pretending to shop, picking up a bottle of tea, etc., just waiting until she leaves, and then he leaves too once the coast is clear.
I'm not saying this is what happened, but I'm saying it is one logical explanation given what we have been told.
