We had a dinghy "borrowed" and set adrift somewhere where it was about 1.5 mi. back to the marina by land. Fortunately, we were with friends, and they gave us a ride back to the marina, and the next morning, helped us find the dinghy.
We have also had a visiting dinghy tied up to us poorly, that went walkabout while we were all below having a chat.
And then there was the time they stole a brand new dinghy off the stern of our
boat, and set it alight. We found the remains, and eventually got new tubes. It is extremely inconvenient to have one's dinghy stolen: one has to go into
marinas, and find something that will do for you for a dinghy. Often no spare ones around. I certainly felt quite angry about it. An approximately $5,000 hit, way back when that was more
money than now, dinghy repair, plus
outboard motor.
I hope your friends get their dinghy back. Sometimes, it is better to say it went missing, rather than was stolen. Ours was found with the hypalon burned up after I put an ad in the paper saying it had been stolen. The policeman said they probably burned it to destroy DNA evidence.