It is the germane point, if they can establish that his tenancy was predicated on his accepting their terms, including a prohibition on pornography. HE was the one in violation, since he had tacitly (or explicitly) agreed that the parents would not accept porn in their house.
The parents have a pretty strong case, depending on what they can prove about any prior agreements on the terms of his tenancy.
That's a big IF, and even if the court finds for the landlords on that point, there's still the destruction of the tenant's personal property to be adjudged.
Bringing his porn collection onto the landlord's property could be judged as sufficient grounds for eviction, but that doesn't give the landlord the right to destroy the tenant's personal property.