Hopefully she thought she was just leaving him for a minute and got unexpectedly delayed inside, and maybe she had had the air conditioner running before she shut the engine off so that the inside of the car was pretty cool.

I try to think of the best possibility, and if the dog was fine, then that's okay. I might have said "Gosh, I saw your dog in there and was worried" when she came back, maybe have a frien "concerned neighbor" conversation.