Fatigue.
Happened to my 1.2s (Brose TF) motor too but it was accompanied with the internal carbon drive belt damage, both separate phenomenons.
What was the model-year of his e-bike?
My friend started getting a weird clicking noise from his brose..this is another one, out of warranty so had a go himself.
Couldn't see anything, put it back together and it's fixed.
Maybe...
He started on the wrong side and it wouldn't come part, rang the dealer and they said.
Don't!!! You'll never get it back together, it was a few mm apart, just pushed it back.
Got away with it.