If you come across this, take lots of photos with your phone, get witnesses and video the dog in distress.
Call the police and ask them how soon they can attend, if they cannot get there within minutes then advise them what you are going to do because you feel that the dog is in great distress. It only takes 15-20 minutes to kill a dog in a hot car.
Pick the cheapest looking window to smash, this may in fact be the windscreen as most cars have windscreen insurance.
Big rock, rescue dog. Job well done.
When owner returns get as much evidence about them as you can, name, photo, their replies etc. Then pass it onto the police & RSPCA to prosecute them.
Before the police do turn up scarper.
If my dog is with me I cannot go into a shop unless it too can. If I have to wait outside then we both wait outside the car.
Simples.