What he is doing is probably illegal then (if he’s rent controlled). You can’t profit like that.
Who is being harmed by a rent-controllee sub-leasing? Literally no one. Because the harm to the owner is already made in the form of rent control.
The owner is harmed, because otherwise they could have gotten a new tenant and raised the rent back up to market rate.
If rent-control didn't have the restriction, then it wouldnt make a dent into rent prices, but it would clearly show that it is a mere transfer from the owner to the rent-controllee.