They are a bit irritating to any woman's stomach, and should be taken with food. Prostagladins are produced but quickly degraded in the body, in about a half hour. She can also gauge the start based on PMS symptoms or the first twinges of cramps. After the start is OK too. There are scores of different medications and brands to choose from.
Different formulations work well on different women.
This is becauese prostagladin production is a whole series of events. There are individual differences as to which level of intervention works best. There is no way to predict which drug will work best on a particular woman. She should simply try one and then a different one, if it doesn't work. Check the back of the bottle for the generic chemical name to make sure the medication is not another brand name for the same drug.