Praise be to Allah.
If a Muslim did not fast at all in Ramadaan, because of an excuse such as travelling, sickness or nifaas (postpartum bleeding), then he (or she) has to make up the number of days, because Allaah says (interpretation of the meaning):
“but if any of you is ill or on a journey, the same number (should be made up) from other days”
If Ramadaan was complete, then he should make up thirty days, and if it was twenty-nine days, he should make up that number. It is not possible for a lunar hijri month to be twenty-eight days.
Some scholars are of the view that he should make it up as thirty days, or fast for a lunar month. It says in al-Insaaf (3/333): If a person has missed Ramadaan entirely, whether the month was complete (thirty days) or not (twenty-nine days), because of an excuse such as being held prisoner and the like, he should make up the number of days whatever it is, like the numbers of prayers, according to the correct view. According to al-Qaadi, if he fasts for a lunar month that will be sufficient, whether it is complete or not. If he does not fast for a month then he should fast for thirty days.
Based on the former opinion, whoever fasts twenty-nine days from the beginning of a complete month or from the middle of a month, if the Ramadaan that he missed was twenty-nine days, this is sufficient, based on the number of days. According to the second view, he should fast an extra day so as either to complete the month or the number of thirty days. End quote.
It says in Manh al-Jaleel (2/152): If a person does not fast in Ramadaan at all and the month was thirty days, and he makes it up during a lunar month that turns out to be twenty-nine days, then he should fast another day. But if it is the other way round, he does not have to fast the last day, because Allaah says, “the same number (should be made up) from other days”. This is the well known view. Ibn Wahb said: If he fasts following the lunar month whatever he fasts will be sufficient, even if the month is twenty-nine days and Ramadaan was thirty days. End quote.
Conclusion: Your wife should make up the number of days of the month, even if it was twenty-nine days.
And Allaah knows best.