Using charting methods to prevent pregnancy is not a very reliable method of birth control and you two will have to be very, very vigilant to chart properly. Ovulation can occur at any time, and a woman is fertile 5 days before and up to 3 days after ovulation. To chart do more research, she'll have to take her temperature daily, at the same time, after at least 3 hours of sleep, if she's feeling under the weather or forgets to take her temperature that will throw everything off. To be really accurate she should check cervical mucus and possible cervical positioning.
And even with all this to prevent pregnancy you should wait until 3 days after ovulation in any cycle before you can be considered "safe" which means there may only be a week or so that is "safe" during the month.
If you truly don't want to get pregnant, use a more reliable method such as condom, birth control pills...Or both together.