ANSWER:

Such questions are created by Satan to occupy people’s minds and mess with them instead of them working hard to reach paradise. 

Ask your best friend to work really hard to get admitted to paradise first and to be confidant that Allah will satisfy all those who are admitted there.

My best friend wants to know if in jannah they can change their gender? He has acknowledged the fact Allāh never make mistakes but was wondering if they could change in jannah??