Can my friend legally sue his trade school?

There is no such thing as an illegal lawsuit.  There are valid and invalid lawsuits. Invalid lawsuits are dismissed by judges on legal grounds before being heard.

Valid lawsuits are not always winning lawsuits. My guess is he (or you, you began with 'I ...got his certificate')  would not win. To win, you have to prove how you were damaged. My guess is language in the original contract gives the school a legal out, or that the pandemic state of emergency gives it a legal out.

He can sue, that part is legal. It will cost him a lot of money, and he will lose. But it is always legal to sue.

He needs to read his contract and see if it actually says what he thinks it says.    There may be a loophole in there that says "space available"  meaning he can do it if there is room in the class.   By shrinking the class there may actually not be enough room.    If he tries to sue now it will take a long time because the courts are on restricted work load too.    He would do better spending his time readying technical books on how to fix cars.  

This is to rally legal. Many clinics are not taking new patients until September. Now that the Oklahoma Republican governor has finally extended Phase 3 foe another 45 days, that probably means no new patients for another 45 days. The Oklahoma governor did have covid 19. I have no idea if he is?still in quarantine. 

Assuming his life isn't over, once Covid passes he can go back to school.

Legally suing as opposed to illegal suing?  And "your friend" investigated the value of this "school," right?  They didn't show him videos and cardboard cars, and he worked on real cars?  These "schools" are a dime a dozen.

Really, Russell?  Really?  What about special precautions doesn't seem legal to you?  As I write this, my husband is in the hospital with Covid.  Right, your auto mechanic friend should sue for his damages.

What are his monetary damages?