My goal is to grow my business (currently 3 rental properties) and work on systems and whatnot, yet I'm deeply troubled by one of my tenants.

What she can negotiate with him is going to be limited to what you allow her - she can't allow him more than what's allowed for her (pets and whatnot).That way I also personally wouldn't care about doing a credit check on the boyfriend.

Document the condition on the day she moves out so you know what not to hold the new tenant responsible for.Don't require the place to be absolutely perfect!

Don't get into "reasonableness" and what not.  

also, you can explain some policies or what not as well.  

If this happens, then at least you will learn what NOT to buy in the future.One more bit of advise, and be warned this may sound harsh but I say it because I think it is what you need to hear: stop blindly believing and following everything you read or hear on BP (or anywhere else).

I'm surprised big government doesn't have a a program where they educate tenants on what not to say when applying to live in a rental. 

Although I do agree with @Nathan G. about walking a fine line with what to do and what not to do.

Gotta raise that rent, even though such raises are no fun when the tenant inevitably calls all upset and what not.

It's those maniacs that buy homes cash via quit claim deed off of craigslist that really get screwed.Make sure your property manager is a licensed real estate brokerage.Google Clayton Morris and/or Morris Invest for a cautionary tale of what not to do when buying turnkey real estateUnderstand you can not eliminate all risk, only mitigate it.