Cumming on command is a pretty hardcore erotic hypnosis technique (hard work but pretty awesome).
The best books I’ve read that deal with it in detail are by a guy called Mark Wiseman – ‘A guide to Erotic Hypnosis’ – they’re really good. Make sure you really take time to learn what and how it all works, you’re seriously fucking with someone’s mind with this stuff, and as his books say, you need to carefully word the suggestions you put in place.
The other way to do it is simply force of will, edging someone and commanding them not to cum, again and again, so it affects them on a subconcious level, and then finally giving them permission. Again, super hard work and takes a long time to build to that kind of intimacy. So yeah, she’s really picked a doozey to start with. But have fun exploring it and don’t get too disappointed if you can’t, it’s highly dependent on personality. Diva dreams of being suggestible enough to do this, whereas others I’ve explored this stuff with have entirely lost the ability to cum without permission no matter how hard they try, and even lost the ability to edge, which was a serious mindfuck.