"We met in York." Johnny was able to say it easily and with no waver of dishonesty. Because for him, it was the truth. "When Jo vanished. I'd just helped Jo out with that thing in the store, when it was being robbed while she was there. She'd been teaching me about hunting. Then she disappeared and Mary appeared. We almost literally ran into one another going back to our rooms, started talking, discovered we were looking for the same person. Kind of became friends."
There were, of course, a few things left out of that description, but nothing that would point at Johnny as a liar. The gist of it was how it'd gone.
"After we found out Jo was back, we just became better friends, and have been looking out for one another ever since."
He parted only briefly to serve a customer and returned to Dean again, ready for more questions and more answers, if it was going to come to that.