1) I mentioned that all the buyer needs to do is to add a language on the contract that "Seller will be responsible to deliver property free of liens and code enforcement violations". That will make the seller responsible for clearing any violations prior to closing.
2) The buyer has no business calling code enforcement on the seller to add pressure on the seller as you stated on your post. That move, in my opinion, is very unethical that will automatically piss off any seller and will break any deal. Only the tenant or at most a neighbor that could be affected should call the code enforcement. The buyer (that is not even under contract) is not affected at all by how the seller is running its property and should not attempt to use this type of tactic to add pressure to the seller.
3) There are many types of landlords. It seems that this seller is focusing on renting to low-income roomies. Or as the buyer claims (people with filth). Perhaps the buyer plan is to rehab the property and charge higher rent to more affluent tenants. However, there is a market for roomies and all they want is the cheapest rent they can afford. They are just one step away from becoming homeless if they are paying rent the cay afford instead of sleeping in the street that works for them. Again, if the tenant feels they are living in an unsafe condition they are the ones that should complain to code enforcement.