[ryandrake]

Or, for an ethical gray-area solution if your guest has only a one way trip (like to the airport): give them a Clipper card with some minimal ($2) balance. They should be able to go to their destination regardless of the fare. The card’s balance will go negative, but they can just throw it away.

[nitrogen]

The BART gates won't let you out if you don't have enough money on a ticket, so you'd have to jump the gate or add funds at a kiosk (think the same applies for cards, too).