Craphead arseholes who did this. It strikes me, that whoever stole it either lives far far away, or intends to use it commercially.

It might be worth sending pics of the stolen trailer to the more established scrap metal collectors/ tow truck operators in the area, to see if they if any spot their competitors using it, they would be sure to report it, small scale steel recycling is a cut throat business.
You might also suggest to your friend to keep an eye on ebay, trading post, circulate details to the regionally local classifieds - the police are unlikely to be that thorough. It is quite possibly still in the local area?

I really hope they get caught, and your friend gets his trailer back.