Giant E Bike rear cassette 8 and 9th gear slipping

[Let's Ride]

Hello,

 

A friend has an E Bike and the chain slips in gear 8 and 9. Bike is not old so I am very surprised the cassette can already be worn.

 

Could it maybe be that the torque from the assistance caused this?

 

He isn't very technical and does whatever the bike shop says... 

[Let's Ride]

Swapped the rear wheel with another wheel and no slipping so either he got given an old cassette or it has worn very quickly.