This is always a hard call, had the same situation, this summer on iPhone 6. I aggree I would check that area for signs it was dropped. If your not sure I would tell the customer you will replace the screen at your cost this one time.
I know it sucks but some times it's not worth the fight. In my situation gave the customer two opitions I would refund him $60 from the first repair or I would replace the screen at cost for $70 (screens were cheaper then) and customer paid the $70. Three months late I fixed an iPad for that same customer, but you have to be careful because you don't want to be working for free.