If someone offer Salah prayer inside the Kaabah, in which direction do they face.
Basically you can pray inside the Ka'bah to what ever direction based on the hadith:
Pray in al-Hijr when you intend to enter the House (the Ka'bah), for it is a part of the House (the Ka'bah). Your people shortened it when they built the Ka'bah, and they took it out of the House. (Sunan abi Dawod)
For further references see also in Sahih al-Bukhari and Sahih Muslim (suggested by @Uma in a comment) and the post I'll refer to later.
Note that there are some narrations of ibn 'Abbas saying a prayer inside is invalid, the main opinion of the dhahiri and maliki school is that it is permissible to pray nafl (optional) prayers inside, but praying fard (obligatory) prayers is not valid and should be redone.
Imam a-Shafi'i has said that if some one prayed inside in the direction of the door of the Ka'bah with no part of the interior between him and the door his prayer is invalid. Else whatever direction he chose for the prayer inside the Ka'bah is a qiblah.
قال الشافعي : فيصلي في الكعبة النافلة والفريضة، وأيَّ الكعبة استقبل الذي يصلي في جوفها فهو قبلة... ولو استقبل بابها فلم يكن بين يديه شيء من بنيانها يستره لم يجزه.
See also Interior of Kaaba
Some references in Arabic: islamweb #29271 and bin Baz #2482