So, now, in addition to inspecting all hitches for vandalism, and inspecting all tow bar pins for vandalism after each stop, we also need to get out the torque wrench and check the torque on all lug nuts. Those vandals with lug wrenches are all over the place!
I suppose it would be a good idea to get the gauge out and check all the tire pressures, too.
While I am sorry to read about what happened to your friend, one incident does not constitute a clear and present danger, and there is no way to know whether the lug nuts were deliberately loosened, or just came loose (I had that happen once, long ago).