I would guess that if it's Grandma's deal, then Grandma is the only one with the ability to take action.
Obviously, their reputation is of no value to them. I would try to make sure they are limited to the letter of their agreement, whatever it may be. I believe that if they take more than what was contracted, damages get up their pretty quickly, something like triple stumpage? Of course, what you can do is controlled be the laws of your state. I think it would be wise to get professional help ASAP, and I would start with a reputable forester. That person could let you know if you have legal grounds to recover any damages. I suspect at this point all you can do is limit what happens from here out.
And of course, advice on the Internet is worth only what you pay for it, our ability to comment is affected by the quality of the data in, there are two sides to every story, and opinions are like a_ _hole_, everyone has one and they all stink (except mine).
Good luck