One tenant damaged other tenant's property while shoveling HELP!

[Nicole Leand]

I own a triplex in Minneapolis, MN. One unit (tenant 1) covers snow removal for the building for a rent reduction each month. My tenant (tenant 2) in another unit just contacted me stating the tenants responsible for shoveling damaged her car while shoveling the driveway (tenant 1 apparently used the car as leverage to pick up the snow and scratched the other tenant's car in multiple places all down one side of the vehicle). Tenant 1 with the car stated she watched her do this technique while shoveling earlier and asked her not to do it against her car. When she went out today to her car after another storm, she had scratches down the side of the vehicle. Am I responsible for the damage caused by tenant 1 because she's technically removing snow on our behalf? Or should this fall on tenant 1 or tenant 2's rental/auto insurance? 

Any suggestions would be greatly appreciated!