If a car bumps into another can it cause an outpouring bump on the other car?

when my husband turned off his car apparently it was in reverse and not park. Consequently it rolled and hit the car behind him. (my husband and the woman were in the dr's office at time of occurrence) At the time my husband and the woman could not find any damage, only an outpouching circle on the bumper. It... show more when my husband turned off his car apparently it was in reverse and not park. Consequently it rolled and hit the car behind him. (my husband and the woman were in the dr's office at time of occurrence) At the time my husband and the woman could not find any damage, only an outpouching circle on the bumper. It appeared to be a perfectly round cirlce, sticking out from the car, not a dent into the car. no scratches near it. The woman said nothing about it. A few days later the woman called and said after talking to her husband she wants my husband to pay for it because it is a leased car. Now her story is that circle bump was never there before the car rolled into it, etc, etc. It will cost $400 to repair it. How can I prove or disprove if my husbands car caused this outpouching circle and not a dent into the car???
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