1. Yes, he was required to declare capital gain in his return if income.
2. No, if he does not have income more than 2.5 lacs, he need not to file return in India
3. NRIs are not required to have Aadhar while filling return.
Please feel free to call/ revert in case you need more clarity.
Thanks and regards
Abhishek Dugar
CA CS B.Com