No one trusts the sick guy. Unless the sick guy is already on the payroll. Tim might not know Sans well enough to be of direct help for him, but he knows him enough to make sure no one tries to subtly off him before he's been cleared to leave. He's all sectioned away in quarantine, but Tim's at least in the unique position to be able to access his prison.

He raises a hand to knock, hesitates, and raps out a shave and a haircut.

Just so he knows it's a...maybe not a friend, but at the very least an ally.