24 Electrical outlets replaced, only 12 work
It sounds like either there's something seriously wrong with a) the products he is using; or b) his methods in doing the work. 

"Back stab" connections are a common source of problems.  These are a little hole you jab the wire into, on cheap outlets. It's easy to mess up and hard to realize it.  Electricians use them because they are cheap and they can work fast and make more money.  The quality is terrible and they tend to fail (which may relate to the question of why the old outlets need replacing).  I never use them.  I buy better outlets, such as the $3-4 ones, and use screw terminals or the "clamp-and-screw" method the better outlets allow. Here's a page comparing "cheap" and "better".
 https://www.handymanhowto.com/electrical-outlets-side-wire-versus-back-wire/

Another mistake is blindly replacing a bunch of outlets without testing each step of the way.  Power comes in from the panel and goes to the first receptacle in the chain, then from there to later outlets, typically by "passing through" the outlet. Understand the layout of the wiring, and work one outlet at a time. 

There's also the mistake of breaking off the "tab" between the outlet screws, or using used outlets where you do not realize the tab is broken off. These separate the two outlets into separate circuits, so for instance one outlet can be always-on and the other can be switched with a light switch.  If you're trying to use both terminals or backstabs to pass-through an outlet with broken tabs, the pass-through won't work and that will kill downstream outlets.