I know this is an old thread, but just wanted to agree with what Sami said about clingfilm. Of course it's best if they wear open shoes after a pedicure, but if they really must wear closed shoes then get them to wait for as long as they're able - then apply cuticle oil all over the nails and wrap in cling-film. We did this where I used to work and it seemed to do the trick in keeping varnish undented (though I always stress to clients that I can't guarantee it'll work perfectly, just in case...)