She could get pregnant from almost straight after. She needs to make sure she is taking enough folic acid, you can get a specific supplement that makes sure she gets it every day, but is also found in cereals, beans, leafy veg and fruit. Not taking enough hasd been linked to spina bifida. She needs to start taking the supplements from as soon as she wants to concieve up until 12 weeks of gestation (this is when the spine will have finished developing, so no holes can appear after this).
There is an increased risk in siblings being born with spina bifida, so the above is very important as although the cause is unknown, this is known to help.
When they are thinking of trying again, they need to ask their doctor about genetic counselling!