One important measure on execution quality is VWAP.
If you friend has a ton of shares which might move the market, a typical way is to hire multiple brokers, each broker to execute a small chunk of sell orders over a period time, or find broker(s) who can cross the trade off its own book or perform an off market deposition on behalf of your friend.
There is also potential legal disclosure requirement prior, during and after the sell of shares as well as tax issues.