Needless to say he's done, but unfortunately so is the damage, and I'm hoping someone might have a creative (and hopefully inexpensive) solution. 

I'm leaning towards that solution with a solid (marble, etc) window ledge that is well sealed.

Which law applies, state or fed, makes little difference.The reality is that simply because you have a applicant that receives preferential treatment does not mean you are obligated to compromise your standards.

Solution: Paid about $500 for a commercial "subject to appraisal (or the value once the renovations are done) which came back at $285k ARV.

The current solution to the issues is pump a few times a year and remove the washer/dryers from all units it appears.

Louis that it's hard to have anyone but the tenant provide treatment anyway and I can't say if your area has similar general practice.

Additionally, I have customers WAITING for me to deliver the solution vs. building the "solution" (which will likely be incorrect) and TRYING to sell it.  

I would just like to add that I don't believe there is a one size fits all solution nor one supreme option that stands above the rest. 

@Jim AdrianWe use a fail amount of polyurethane foam, but have found it is not always the best solution for older properties (Edwardian or earlier). 

Lara Fobian, GRIInvestor (Home Freedom Properties)Realtor® (StepStone Realty LLC)“Solutions-Based Real Estate”