That's a little harsh. Apple pay didn't exist when the 5s came out so it's not like he/she could have taken that into consideration at the time. Many people mistakenly believe that any of the models with TouchID support Apple Pay. Just because someone doesn't know every technical aspect of their phone doesn't mean that they just blindly bought it because of the brand name.
I don't blame them for not taking that into consideration at the time.
It's the fact they tried to add their card to Apple Pay on a 5S, when they should have known their phone wouldn't allow them to do that due to lack of hardware.