I suppose the proper thing to do is summon a locksmith. Especially in those cases where it's a more serious lock involved, than the one in the picture. And be sure to re-lock the offender's bike after releasing yours, to avert any possible claim against you. But I'm not sure how much that would cost (getting your front door opened, out-of-hours, is expensive!), nor how you could possibly recover that expense.
Having said that, my guess is that this was a perfectly innocent mistake. Never happened to me, though once I
nearly perpetrated this on someone else's bike (caught myself in time!
). Perhaps the answer is: call a locksmith
and leave a note on the bike with a copy of the locksmith's invoice, hoping the owner will be mortified enough to cough up.