They did the farewell tawaaf and travelled on the 11th, and delegated someone to stone the Jamaraat on their behalf on the 12th
Question

                There are some pilgrims who, when they had stoned all the Jamaraat on the 11th of Dhu’l-Hijjah, hired someone to stone the Jamaraat on their behalf on the 12th, and they went straight after that to Makkah and did the farewell tawaaf, then they went to Jeddah and boarded the plane, because their tickets were booked at that time only and they had no choice but to go. Is their Hajj valid? Do they have to offer a sacrifice? How many sacrifices do they have to offer?.
Praise be to Allah.
      If the situation is as described, their Hajj is valid but 
      it is lacking to the extent that they missed out by departing before 
      staying overnight in Mina on the night before the 12th, stoning 
      the three Jamaraat after the sun passed the meridian on that day and 
      bidding farewell after that. Undoubtedly they sinned by doing what they 
      did, which goes against the words of Allah (interpretation of the 
      meaning): “And perform properly (i.e. all the ceremonies according to 
      the ways of Prophet Muhammad صلى الله عليه 
      وسلم), the Hajj and ‘Umrah (i.e. the pilgrimage to Makkah) for 
      Allaah” [al-Baqarah 2:196] and the words of the Prophet (blessings and 
      peace of Allah be upon him): “Learn from me your rituals (of Hajj).” They 
      have to ask Allah for forgiveness and repent to Him from that. According 
      to the correct scholarly opinion, each one of them has to offer a 
      sacrifice of an animal that is acceptable as an udhiyah for not staying 
      overnight, another animal for not stoning the Jamaraat, and a third for 
      not doing tawaaf al-wadaa’, because they come under the ruling of one who 
      did not stone the jamaraat, because they departed before doing so; and 
      they come under the ruling of one who did not do the farewell tawaaf, 
      because they did their farewell tawaaf before completing their rituals. 

    And Allah is the source of strength; may Allah send blessings 
    and peace upon our Prophet Muhammad and his family and Companions. End 
    quote. 

    Standing Committee for Academic Research and Issuing Fatwas. 

    Shaykh ‘Abd al-‘Azeez ibn ‘Abd-Allaah ibn Baaz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan