I am looking at the safety issue here only. All the other opinions about what we would do or not do, if we would leave our children in a car in the garage, or let them sleep, etc, etc, are irrlevant and cloud the issue.
#1 A child should not be left unattended in a car. #2 No child (or living thing) should be left in a car with out air conditioning in 89 degree heat because the temp will rise to over 120 degrees in a matter of minutes and they will die.
I would just be very frank with her, nice, but frank. Tell her that she should not leave the baby unattended to socialize and that the baby should not ever be inside a stopped car without the air on, because that is deadly. It is also not legal.
I like the suggestion that you would gladly go to the car and socialize with her while the baby sleeps, and maybe, you could offer to find a time for play group that is not during the baby's nap time? She probably needs the outlet, and you need to be firm about safety.
If your husband were to have called 911 when he saw a baby in a car, I can tell you exactly what would happen, because that happened to me. I was grocery shopping with my two year old, and it was very hot, and I had a young man taking my groceries to the car, so while he put the grocerieis in the back of my suburban, I put my two year old in her car seat, I started the car and turned on the air, gave her a bottle of water, turned on her tv and then and went to the back of the car with his tip. As he shut the back door, the car locked. Not sure how, but it did. I called my husband, he had his keys and was out of town, so I called my father in law, who relayed the information about my car to a lock smith, and was on his way to bring me a new key within 15 mintues. I never left the car, and my child was sitting in her seat, drinking water in the air conditioning, watching the Aristocats. The man who put my groceries in the car thought it might be helpful to call the police for help once he got back into the store. He failed to tell them that the car was running with the AC on and that my daughter was never unattended or that I had reached a lock smith and had a key on the way. Every emergency vehicle within ear shot of the call pulled into the lot with full sirens blaring, and two tow trucks with cars attached pulled in brakes screeching, and all of them came running with crow bars to bust out my child as quickly as possible. The poliece were ready to arrest me, thinking I had left my child in the car to go shopping! It took a minute or so for their adrynilen to come down as they all came ready to whitness a tradgedy of the worst sorts, one that they had seen before. It is an emergency, and had your husband called the authorities, it would have been treated as such, and yes, she would have been arrested if the authorities found the baby in a car without her being there.
M.