For example the List for Apartments would be to target "Landlords with Code Violations list."  4.

Make sure it explicitly explains the process for termination if you are unhappy with their services, but especially if they violate the terms of your agreement.3.

If you don't, they will likely refuse the loan as it presents a 'risk' factor they don't want to deal with.Your should inquire what it takes (time, cost, etc) to get an after-the-fact variance and possible consequences of not getting it (e.g. violation notice, not getting occupancy permit, ??).

Dealing with a tenant that has violated your lease rules is treated no different when addressing the ESA scammers. 

Have they violated any contractual obligation in a written lease?

If they do not it is a violation of the lease.

Am I committing any sort of rules violations by commenting on a friend's experience with this guy?

If she has full rights of tenancy, she would be able to invite people into the home as long as she was not violating the terms of the rental agreement with the property owner.

Usually the real problem tenants are in violation of their lease if they have one... so we told them if they signed our lease we'd work with them and give them some time (30-45) days to get out instead of moving to evict.The deferred maintenance will start showing up immediately after purchase, you'll learn a lot about the existing tenants based on how they deal with the deferred maintenance issues when you take over as a new owner.

It is your choice to accept or deny the application, however not letting someone fill out the application can be considered a violation of Fair Housing Law.