Most thefts are preventable. Sounds like your buddy put it in a spot where it was easily hauled off, didn't lock it down, and had no plan for IF it ever got stolen. That's a recipe of "oops" right there.
My trailer goes inside the garage every night, with the truck detached but in front of it. For them to get the trailer out, they have to move the truck (which means hotwiring), open up the garage and pull the truck out, turn it around, then back up into it. Then, they have to cut the chain holding the trailer to the garage floor anchor, hook it up, and get it out of there in one piece. Not easy, and even less easy as it's only 25 yards from my back door and where my Labrador is sleeping with good ears.
When they DO get it out of the garage, I have a GPS tracker in the trailer so recovering the goods will be easier, at the very least it will help to arrest said scumbags.
Here's a good site with trackers:
Insurance is for "all else fails", but I try very hard to make theft difficult enough that it's not worth it for the crooks.