The area of law that he would need to be aware of with regard to the stock sales would be "securities law."
A security can be a: stock, which speaks to part responsibility for organization; bond, which resembles an "IOU" from an organization to an individual; or an alternative, which is the privilege to purchase something later on at a foreordained cost. Companies, governments, and people purchase and offer securities as a method for contributing and developing their cash.
Choosing which sort of security to purchase and from which organization requires a decent arrangement of research and examination.