who should pay?

We asked MIL to mind our dog for two days. she usually does this and offers to take him to the beach for a run as we are busy with our kids. Mil keeps her guitars and music equipment in her garage as well as her car etc and other things in storage. She usually keeps our dog in the yard or when raining in the laundry. This particular time she has let the dog in her garage. The dog has run towards the glass door to the front yard and smashed into it. He had to run past her precious music equip also to make it to the door. He damaged the glass and she wants my man to pay for the glass. Who do u think should pay? I think she was stupid to let the dog into the garage at all ....like why??