RE: To good to be true...
I'd have her (or you) sign a bunch of posted signs and get them up ASAP!! I had a similar situation a few years ago and now I control that piece of land. It wont' take too many times for them to be busted especially with posted sign up to chase them off. Sounds like it could be a nice lil hideaway for you. If you run into them politely discus the matter and how you have permission and the landowner doesn't want anyone else there. Also, try to contact the son that hunts it too and team up with him. good luck.