The scholars (may Allah have mercy on him) differed with regard to the ruling on udhiyah: is it obligatory or Sunnah muâakkadah (a confirmed Sunnah)? The correct view is that it is Sunnah muâakkadah for the one who can afford it, but there is no sin on the one who does not do it.
For more information, please see the answer to questions number 36432 and 41696.
If a person does not offer udhiyah although he can afford to do so, he is missing out on a lot of reward that results from offering the sacrifice, but he is not sinning, and he does not have to make up what he missed in years past, because udhiyah is Sunnah and the time for it has gone. This is the case if he did not make a vow to offer a sacrifice, in which case it is obligatory for him to fulfil his vow and offer udhiyah.
It says in al-Mawsooâah al-Fiqhiyyah (34/46): If a person does not offer a sacrifice before the time has passed, if it was a voluntary sacrifice he should not offer it; rather he has missed the opportunity to offer a sacrifice this year. But if it was a vow, he has to offer a sacrifice and make up what he was obliged to do. End quote.
Based on that, if she wants to offer udhiyah, then from now onwards she has to prepare ahead of time, so that when the next Eid al-Adha comes, in sha Allah, she will have prepared her sacrifice beforehand, so she will not have missed the time for udhiyah, as happened in the past.
We ask Allah to help us and you to do that which He loves and which pleases Him, and to accept righteous deeds from us and from you.
And Allah knows best.