Hi,
It depends on the bgv team because in my org, the bgv conducts verification of previous employment based on facts/employment details submitted by new joinee in joining kit/hr system. BGV team do not focus on any employment gaps as "they assume" it would have been cleared/explained by candidate during hr interview.
However, if your friend's personal reason is linked with any medical issue, bgv team would expect medical reports or if he was associated with third party company or small startup firm then they would ask for references or perhaps relieving letter.
Thanks and Regards,
Akash Sharma