I pray to Allah to accept your parent's hajj and make it a source of expiation for them.
Tawaf al-wada' is wajib according to the majority of scholars; and those who deliberately missed it should expiate for it by offering a sacrifice; it is preferable to do the sacrifice in the haram to feed the poor there. They should do it there, if they can. If, however, they are not able to do it there, they may do it in their home country and feed the poor.
Having said this, I should also point out: Eminent scholars agree that it can be excused in regard to those with valid reasons such as women who are menstruating or those who are sick. Therefore, if your parents were prevented from doing it because of circumstances, they had no control over, then they need not worry as they can be excused.
Alternatively, they may also choose to follow the ruling of Imam Malik and Shafi, who consider tawaf al-wada' as a sunnah and not wajib; thus, according to these imams, they need not worry about their mistake, and there is no need for them to offer a sacrifice for the omission.
There is nothing wrong for your parents if they choose to follow this opinion-even if they are adhering to another school such as hanafi or hanbali. As ruled by authentic scholars such as Shah Waliullah and others, a person following a particular school can follow a valid ruling of another school for legitimate reasons, and your parents can be deemed to belong in this category.
In conclusion, your parents may choose to a sacrifice to expiate for the omission, or simply ask forgiveness of Allah.