Hi,
No, he does not. If you brother in law is not a UK resident, then there is no need for him to register for a National Insurance number or Self Assessment.
There is a Capital Gains Tax calculator to help work out if there is a gain here:
Tax when you sell property
As a non UK resident, he must report the disposal of the property within 60 days of the completion date, whether there is a gain or not. He can print and post a paper form here:
Report your Capital Gains Tax on UK property by post
Thank you.