Apartment HOA probably have different restrictions than single family home subdivisions HOA. The best way to figure it out would be to look at a copy of the covenants for that specific development. The letter can only be trying to enforce the covenants.

Your friend should have received a copy of the Home Owners Association and covenant rules when he signed the lease. Those rules may have been changed by the members of the HOA committee. He needs to get a current copy and study it carefully. If he believes he is not in violation he should appeal to the HOA with an objective and studied case. If the HOA doesn't accept his appeal, he should ask a lawyer to review the rules and the letter.