Locked Car, Where's The Key Battery?

I recently saw a distraught young lady weeping beside her car.

"Do you need some help?" I asked.

She replied: "I knew I should have replaced the battery to this remote door unlocker. Now I can't get into my car. Do you think they (pointing to a distant convenient store) would have a battery to fit this?" 

"Hmmm, I dunno. Do you have an alarm too?" I asked.

"No, just this remote thingy," she answered, handing it and the car keys to me.

As I took the key and manually unlocked the door, I replied, "Why don't you drive over there and check about the batteries. It's a long walk."