My Grandaughter was paid child support by her father, however she never received the money.?

My Granddaughters father has been paying child support for some years, but my grandaughter never received any money because while she was out of town several checks came and mother cashed them, so when she went to see why she wasn't receiving any more checks, she was told when they could prove who was the real person that checks belong too then she could receive the money. What does she do about this?