Has she been mated before successfully? If she was mated and successful in the mating then she should be good to fertilize any eggs she makes from now on. In insects when they mate usually the male gives the female a bunch of sperm which the female stores in a special storage place called the spermetheca, which she uses to fertilize the eggs as she lays them. So one mating is good as long as it was a successful mating. But it is hard to tell sometimes in mantises if the mating was successful or not.
So even if you mate them and the next day your female lays an ootheca it should be fertilized.