Before you say or do anything, it's important to realize – and respect – that you can't make somebody do something that they don’t want to do. If a gal – or guy for that matter – isn't into something sexually speaking, then she's not, and hopefully she's made the best decision for herself in opting out.
You can, however, suggest self-pleasuring as a means of sexual exploration, whether partnered or solo. In doing so, your idea is going to sound much more appealing if you point out the health benefits of masturbation, such as the fact that it's a great form of stress relief. There are the sexual benefits as well, like that it increases sexual awareness and can enhance your sex life with more intense orgasms.
You can also give the person a book on solo sex, like Betty Dodson's "Sex for One", which takes on a lot of the myths and taboos around solo pleasuring. If you're intimately involved with the woman, you may also want to let her know your personal feelings about masturbation, including how it sexually excites you, if that's the case. No matter what, make sure that you're not being critical or demeaning in your efforts. Self-pleasuring isn't for everyone, and that's okay. People may, however, simply need to be enlightened about how this is a perfectly normal, natural means of sexual pleasuring and satisfaction. Information may just invite exploration.
Thank you for your question and good luck!