This is a fun one. I don't have any particular domain knowledge about this question. So this one is just a guess.
My answer is strictly from a practical standpoint. If I were faced with this situation in real life, what would I do? (Technical point: I feel the vagueness of the language, "Is he allowed to" allows me to answer this way.)
My assumptions are that:
- The cost of consulting an attorney on the matter or filing a law suit would likely exceed the combined total cost of the bike and the lock.
- Usually, the law follows what "feels right" and what makes common sense to the average person. Usually. Not always. But usually. (Legal principles: "Equity follows the law." and "Equity does not aid a party at fault." See this reference.)
What I would do:
So, I would do the following... (if I were in the U.S.)
I would simply cut the bike lock and repossess my bike (unilaterally) if and only if all the following conditions were true in the situation:
- I could confirm without any doubt that the bike in question is actually my bike and not just another one that looks just like it.
- I could not find anyone around who looks like they might be the owner of the lock or the other bike. If I could find the owner of either the lock or the bike it is highly likely there was some mistake and the situation could be resolved directly with them.
- There are no law officers nearby. If so, I would engage them in helping me rectify the matter. If they said it was a "civil matter" and refused to get involved, I would proceed to the next item on this list.
- I had the tools handy and available to cut or break the lock.
If any of the above conditions were false, I would flag down the nearest law officer or call one to the scene to help resolve the issue. Any other approach would seem impractical to me on the basis of my above assumption numbered 1.
If I were anywhere outside the U.S., I would involve the local authorities without considering the unilateral repossession option.