My friend have problem in hair ...he have gap in his head..so how to grow hair in that gap? which food want to eat? which oil is to use it?

None of that: Due to two possibilities - male pattern baldness developing or a localized fungal infection. Either way, see a local doctor (MD, western-trained, respected - not just any doctor in India). If due to infection, medicine can be given. If developing male-pattern baldness, can use medication for hair growth or get hair transplant. No food or oil will help this case.