What can you do when your heel keeps slipping out of the back of a shoe?

My mom just bought these peep toe wedges and the are a little to big ... the size smaller was too small so she had to go with the ones that were a little bit bigger ... the only problem is that when she walks her heel comes out of them ... the material is canvas and the sole is some kind of hemp and like i said they are wedges so they do have some height, what can she do/use to keep the shoe from slipping off ?