Praise be to Allah.
The fuqaha’ differed concerning the time when the sacrificial animal is to be slaughtered by those who are doing tamattu‘ (‘Umrah followed by Hajj, exiting ihram in between) and qiraan (‘umrah followed by Hajj, without exiting ihram in between). The majority are of the view that the time for offering the sacrifice begins on the Day of Sacrifice, and it is not valid if done before that.
The Shaafa‘is are of the view that it is permissible to sacrifice it after entering ihram for Hajj, or after exiting ihram for ‘Umrah and before entering ihram for Hajj, according to the more correct opinion.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (3/247):
With regard to the time for offering (the sacrifice), it is the Day of Sacrifice. This was the view of Maalik and Abu Haneefah, because before the Day of Sacrifice it is not permissible to slaughter the udhiyah (sacrifice), so by the same token it is not permissible to slaughter the hadiy (sacrificial animal) for Hajj tamattu ‘ during that time, as it is also not permissible before exiting ihram from ‘umrah. Ash-Shaafa‘i said: It is definitely permissible to slaughter it after entering ihram for Hajj. With regard to the time before that, after exiting ihram for ‘umrah, it may or may not be permissible. The reason why it may be permissible is that it is a sacrifice that is connected to ihram, and may be made up for by fasting. Therefore it is permissible to slaughter it before the Day of Sacrifice, like the compensatory sacrifice to be offered if one puts on perfume when in ihram or puts on tailored clothing. Because it is permissible to make it up before the Day of Sacrifice, it is also permissible to slaughter it before that day, like all other fidyahs. End quote.
An-Nawawi (may Allah have mercy on him) said: To sum up, there are three scholarly views concerning the time when it is permissible to do it:
1. After entering ihram for ‘umrah
2. After finishing ‘umrah – which is the most correct view
3. After entering ihram for Hajj.
End quote from al-Majmoo‘ (7/184)
The most correct view is that of the majority.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said, when discussing the conditions of the hadiy (sacrifice):…
4. It should be slaughtered at the proper time for slaughtering. Concerning that there is a difference of scholarly opinion, which we may sum up as follows:
The first view is that the sacrifice of tamattu‘ should not be slaughtered except at the time when the udhiyah sacrifices are also slaughtered, which is the day of ‘Eid and the three days following ‘Eid.
The second view is that it is permissible to bring forward the sacrifice after having entered ihram for ‘Umrah. So the hadiy may be slaughtered even before going out to Mina for Hajj, because it is permissible to fast – for the one who does not have a hadiy – before going out for Hajj even though it is making up for the sacrifice. So if that is permissible in the case of making up, then it is more apt to say that it is permissible in the case of the original action. This is the well-known view among the Shaafa‘is.
The correct view is that it is stipulated that it should be done at the right time, and that the hadiy in the case of tamattu‘ can only be offered on the days when the sacrificial animals are to be slaughtered, namely the day of ‘Eid and the three days that follow it.
The evidence for that is that if it was permissible to bring forward slaughter of the sacrificial animal before the day of Eid, then the Prophet (blessings and peace of Allah be upon him) would have done that, but he said: “I will not exit ihram until I offer the sacrifice.” And that is only permissible on the day of Eid.
End quote from ash-Sharh al-Mumti‘ (7/91).
If the pilgrim relied on a fatwa that permitted slaughtering the sacrificial animal on the day of at-Tarwiyah, then his sacrifice is acceptable and he does not have to do anything further. If a person has to offer a compensatory sacrifice, he may appoint a proxy to slaughter it on his behalf in Makkah, and he does not have to slaughter it himself or send the cost to Makkah.
To sum up: your friend’s parents do not have to do anything further, and their sacrifice is acceptable, in sha Allah.
And Allah knows best.