In Islam, masturbation is a sin and prohibited for both males and females. It is also one of the invalidators of fasting, should it reach sexual peak. If what you mean by no vaginal discharge that she did not have an orgasm, then her fast would be valid. However, if she had reach to that, and if Ghusl Janabah becomes wajib on her, then it would invalidate her fast. She will hence need to repent, re-do that day as Qadha', and also pay Kaffarah of intentionally breaking fast.
Ways of avoiding such an act are:
1. Focus on your level of taqwa and become more consciously wary of Almighty God in everything you do and wherever you are, especially when you are alone.
2. Increase in your practice of Dhikr of Allah ta'ala, especially when you wake up, and before you sleep.
4. Stay away from anything that provokes lust, and anything that brings about sexual excitement.
And Allah knows best.