He wants to leave Makkah then come back to do the farewell tawaaf when the crowding is less severe

It is not permissible for any pilgrim to leave Makkah before performing the farewell tawaaf. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: Is it acceptable for those who live in Jeddah to leave Mina and go to Jeddah without performing the farewell tawaaf, then come back a few days later to perform the farewell tawaaf? 

He replied: 

It is not permissible for the people of Jeddah or anyone else to go back to their cities before performing the farewell tawaaf, then come back to Makkah when the crowding has become less severe. They should not leave Makkah until they have performed the farewell tawaaf, because the Prophet (peace and blessings of Allaah be upon him) said: “No one of you should leave until the last thing he has done is to circumambulate the House.” And Ibn ‘Abbaas said: The people used to depart from many locations (within Makkah), but the Prophet (peace and blessings of Allaah be upon him) said: “No one of you should leave until the last thing he has done is to circumambulate the House.” 

Majmoo’ Fataawa Ibn ‘Uthaymeen, 23/353. 

Even if he comes back after that to perform the farewell tawaaf, it will be of no benefit to him. Shaykh Ibn ‘Uthaymeen said: 

If he leaves Makkah heading for Jeddah, and reaches Jeddah, then even if he performs it (the farewell tawaaf) it will be of no benefit to him, because he left, so how can it benefit him after he has left? 

Majmoo’ Fataawa Ibn ‘Uthaymeen, 23/353.