I am curious what will happen to his business. If the owner didn't have a partner, the business would either be sold or go out of business and the assets sold to cover liabilities (including the tax man.)
Are businesses typically included in your personal will? Absolutely.
Would the business typically have a succession plan in place for such an event? If there is a partner in the business, they should have had a partnership agreement outlining how to handle the death of either partner.
The only thing I know about the business is it is broadly categorized as specialty engineering. Sometimes such business' biggest assets are intangible--like intellectual properties and their customer list. Hard assets would be working capital, computers, office furniture, etc., and they're easy to place a value upon. It's the intangibles that are difficult to place a value on.
Hopefully some of the employees would step up to purchase the business from the estate. These are difficult situations to place an executor of the estate.