Again, I'm no lawyer, but INTENT has a lot to do with legal interpretation and prosecutions. If your intent today is to give a friend an aerial shot of their property, then you have no intent in breaking the law. As you said, it's just a kind gesture, not work for hire or compensation, so no laws broken thus far.
If years later someone else were to use that photo for THEIR own intentions, i.e., as a rental shot, then I do not think any Court would find you culpable of breaking FAA regs or rules. It would not, in my mind at least, retroactively make you guilty of breaking any laws when you took the photo. Going after you would be like going after a gun manufacturer instead of the idiot who used it to kill someone.
I also do not think the person utilizing the photo as stated would face any prosecution, either. Obviously I could be miles off here, but I do not think I am, and I don't think it would even raise any eyebrows. Having said that, Government is Government.