NAH.

If you don't see a handicap placard or a visible obvious disability, I think it's reasonable to assume "oh maybe they didn't notice it was a disability parking". You merely pointed it out to the lady, and she answered accordingly.

My dad is disabled. He has a placard in his vehicle at all times. Sometimes when he drives mine, he forgets to transfer the hanging card/placard, but he always carries his wallet card if he ever needed to show proof. So sometimes he'll drive my vehicle, park in a disability spot, and I'll jump out from the passenger seat to grab what he wants/needs from the store.

The lady saying her mom is in the backseat is totally reasonable. Sometimes you just need to be in and out of the store, and parking closer to the store makes that easier to do, especially if the disability is chronic pain related and time is of the essence.