He would first need to send the certificates to the transfer agent to hold them in a dividend reinvestment account. Once the transfer agent has the shares in an account, he can then sell them through the transfer agent. I worked for a transfer agent in the late eighties. At that time, the maximum amount we could sell at one time were 99.99 shares of stock.
If the shares were issued as the result of either ESOP (Employee Stock Ownership Plan) or any other employee plan (again I remembering information from decades ago), he would need to contact an accountant regarding figuring out the price at time of issue or if he kept the statements that were sent to him at the time of issue, that would help. Just Google tax basis per share, to help with any capital gains on the stock.
If he does send them to the transfer agent to sell for him sent them either certified or registered mail return receipt requested. Also make sure he insures the stock too. If the stocks were lost, there would be a cost to replace them.