I think he did not get any cashback for education transaction as it is mentioned in terms and condition page.
For rest of the things he got cashback.. And yes you are right about refund thing because he might have refunded it later which might not have been settled on bank end and will get adjusted in next statement.
Jinko lagta hai previous month ka cashback nehi mila please check hdfc old mobile banking page not the reward redemption page. It may appear late in reward redemption page.