Sure sounds like his slider was not locked in the forward position and was sliding back and forth as he drove and/or stopped/reversed.
NOT a good situation - some of the impact loads on that hitch and the fiver pin box will probably exceed the design limits.
If taken to extremes that COULD result in a breakaway.
You need to talk to him ASAP to figure out what was going on and what he needs to do to stop it.
You didn't say what hitch he had - is it possible he also has a Superglide, but the trailer is not equipped with the wedge plate on the pin box? That would result in a similar situation.