
Quoting
adjusterjack
They wouldn't have to "know." All the client has to do is say "Hey, you had a friend working for you, you breached the contract, I'm not paying."
Then it's up to your brother to take the time and spend the money to sue the client and convince a judge that he didn't hire a friend or that the contractual provision is unconscionable.
That puts your brother in a world of hurt for the months that it takes to litigate.
Never accept a troublesome contract with the idea that you can breach it with no consequences just because you think it can't be enforced.