One thing he may not already know is the location of the block drain. He's got to remove this drain plug if you expect to get the green stuff out of the block. Otherwise, he's wasting his time and your money.

Furthermore, the green stuff lives in the heater core loop and he's got to flush the heater core with a garden hose adaptor while the block drain is open. Otherwise, again, he's wasting his time and your money.

Finally, the system is never completely devoid of fluid when you button it up. It might only take 9 quarts. So, if you add a 50-50 mix, you'll be light on the anti-freeze when he's done. To avoid this, you'll need to add two quarts of pure anti-freeze and seven quarts of the 50-50 mix. At that point, the system has the proper amount of antifreeze. If it still needs more fluid, then you add distilled water to top it off.

Dropping these vehicles off at the mechanic and picking them up later is usually a recipe for disaster, unless the mechanic is very good.