That is truly a bizarre story. I've always held the same belief about not taking anything from a cemetery, burial site, etc...
Yes, it is absolutely possible for a spirit to visit someone in a dream. That is one of the most common forms of spirit contact.
What puzzles me a little is what significance that dime could have had. Considering that dime was not from her era, I would not have thought she had an attachment to it. However, I have heard of people leaving small tokens for the dead, coins being an example. Perhaps a curiousity seeker or another ghost hunter "gave" that coin to her as a token of appreciation, rememberance, etc... That is the only theory I've got on it, maybe someone else has another idea. Regardless, it is an intriguing story!