Lock it properly, keep an eye on it, and if you can't do the both, go home or travel to such occasions with a bike that is un-theft-worthy or you don't mind (too much) having stolen.
On a Brompton there are two plainly viewable serial numbers engraved by the factory on both the main frame and the folding swing-arm.
Pity your friend didn't take these down, (irrefutable proof of ownership) and pity they didn't chase down the (their) potentially stolen bike.
Lock it properly, keep an eye on it, and if you can't do the both, go home or travel to such occasions with a bike that is un-theft-worthy or you don't mind (too much) having stolen.
On a Brompton there are two plainly viewable serial numbers engraved by the factory on both the main frame and the folding swing-arm.
Pity your friend didn't take these down, (irrefutable proof of ownership) and pity they didn't chase down the (their) potentially stolen bike.