when she pulls herself forward using her legs.
This is a good insight. Have the client practice rowing w/her feet out of the straps. Not only will this reinforce good technique anyway, it'll prevent her from pulling herself back up the slide with her toes, which would definitely be causing extra work/stress on her shin muscles.
It should be your heels that are attached, and it's less of "pulling yourself forward" and more of "folding yourself up" to get back to the catch.
It'd be interesting to hear if there's a difference from trying this.