I know it's a dick move to lock their bike to yours but presumably it was an accident. Seems pretty harsh just leaving their bike totally unsecured. I don't really know what the alternative is if you don't have time to wait for them to turn back up.
I think getting your bike nicked is a pretty high price to pay for not paying enough attention. Getting your bike nicked because you're using it to try and nick someone else's is obviously different.
I know it's a dick move to lock their bike to yours but presumably it was an accident. Seems pretty harsh just leaving their bike totally unsecured. I don't really know what the alternative is if you don't have time to wait for them to turn back up.