if your wife continously give the formular to your baby after breastfeeding, eventually the milk supply will drop a lot and she will not have any breastmilk, it is the same case of my friends, each time they feed baby and then give baby formular, so baby will full and there will be less stimulation.
tell your wife to trust herself, her milk should be enough for your baby. if she really feel tired, try to pump her milk out and you can feed your baby by cup . but the best is to directly feed the baby, more stimulation, more demand, more milk.