Leave baby in the care of MIL who wishes to stay in the car

My MIL says she wanted to stay in the vehicle car and to wait inside the car with the baby at the car park while we (the parents) go visit the temple. It is suffocating to stay in the car. I read about article adult accidentally leave baby in car and compromise the safety of infant. MIL says she could wind down the car windows & sit in the car with the baby. I really doubt it. She can take the stuffy air, but how can I as a parent allow my baby to subject to risk mths ? How can I enjoy a peace of mind and leave baby in her care ?!