*sigh* Maybe you could print out some literature on chained dogs and give it to your friend. You know... the stuff that mentions how chained dogs are 10 times more likely to turn aggressive.
Yay! That's so cool that Marley is fitting in to your household! :) (at least there's very little chance of her being turned in to the pound :D I was so worried about that.)
Are you keeping an eye on local newspapers to see if someone else is missing her? (and petfinder.com, dog detective, k9amberalert, etc?) Have you posted any ads? (suggestion would be "found female puppy, dark colored, call and describe.") Like you said, she seemed very well cared for.
http://petoftheday.com/talk/showthread.php?t=131562