There was a slight wince and shock at the woman; it wasn't exactly anything new for the youngest Winchester, but it was off putting that a human did this to another human. There had been no evidence of supernatural activity in this place.
Rob did check on the woman enough to know she was alive, and Max did seem to have everything in hand. Johnny's decision and possible demand didn't sit well with the Man of Letters. He wasn't going to let the cowboy go off on his own, certainly not when his sister's life was on the line.
"Yeah, you're not going alone." It was a rule that the Winchesters broke all the time, but that didn't mean he would let some guy go looking for Mary without him. No. Family should find family. Something Max had said about the man was rolling around in Rob's brain, so that certainly added another layer of drama and whatnot to the situation.