This particular McDonald’s is a liminal space, connected to multiple dimensions and timelines like spaghetti tangled around a meatball.
Dude and his wife have been finding and losing each other for centuries.
If you go inside next Thursday it’ll be 1993 and you can watch them meet.
It’s very romantic, but quite crowded, due to three hundred years’ worth of mildly curious time travelers showing up.
Also they run out of Big Mac sauce.
You will probably trip over the briefcase of a businessman from 2067 and get bitten by someone’s poorly-behaved pet robot archaeopteryx, and the intrepid explorer from 1672 in a steampunk dimension will whap you over the head with her umbrella right when he says his first words to her, and your Big Mac won’t have any special sauce.
Also there’s a small but nonzero risk that you’ll step out into the Upper Cretaceous and be eaten by a confused adolescent T-rex that really only wanted your soft-serve ice cream, but isn’t complaining about the rest of you.
Anyway, the guy in the other window has been living in the McDonald’s for six years straight, after his home dimension was over run by parasitic space wasps.
He’d leave, but every time he tries he comes out into either a Category 4 hurricane or the opening scene of the Star Wars Holiday Special.
He’s got his own secret stash of the Big Mac sauce.