His late is not late and he does not have any past due rental amount.
He needs to check the terms of his lease agreement. Under many leases, unpaid late fees become due and payable as rent. He is required to pay these, regardless.

For some reason, the collection's guy of the complex thinks it’s legal to curse out anyone not on the lease.
Of course it's legal. Rude and unprofessional, to be sure, but there's nothing illegal about it.

With respect to the landlord's threat to change the locks, no, that's not legal. "Do it yourself evictions" are not permitted in the state of Missouri. The landlord may not change the locks or enact any other eviction procedures without a court order. Rules are here.