There are many, many problems here.
> Collusion is also not bad unless it's illegal
Did it ever occur to you that laws might be passed despite those same laws not being beneficial? In your model of the world, where legal actions are all, by definition, good, how would legislators decide whether or not to enact a prospective law? If we repealed all our antitrust legislation, can I assume you would agree that employer collusion to hold down wages used to be bad, but was now good?
> no, unions are not a conspiracy. A conspiracy is pretty much defined as such when everyone involved constantly tells each other "don't tell anyone"
I have to agree with the bare semantics here, but... who's alleging that the harm from a bunch of companies secretly agreeing to hold down wages comes, entirely and only, from the fact that the agreement is secret? They do it in secret because it's illegal, and your parent is wondering why employees are legally allowed to do openly what employers must do in secret. Why is there supposed to be a difference? Is there a justification for punishing the companies, other than the fact that their conduct is illegal?
Nothing can be bad because it's illegal; belief following that reasoning is religious, not valid.