CA Real Estate
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You would hire a real estate attorney to handle this for you. I may be able to help you with this as I work statewide.
In a legal sense, Rob has a revocable license to plant and harvest peach trees on your grandparent's property. He has a right to plant and harvest peaches, and to go onto the land for the purpose of maintaining the trees and harvesting the peaches. But Rob's rights are not permanent. Yes, you can negotiate anything with Rob to change the verbal agreement for the peach tree license. A bare license is revocable at any time. You want to assert your grandparent's rights so that Rob does not gain more rights of an easement. There really should be a written license agreement between your grandmother and Rob and in that agreement, Rob can pay a license fee for the use of the land. Furthermore, if Rob hasn't paid his 20% of income from the peaches, then your grandmother can sue for recovery of that money... but only for the past two years since the statute of limitations on verbal agreements only lasts two years.
One other option for your grandmother would be to get a reverse mortgage to help pay for her health care costs, but that has to be refinanced after she passes if you want to keep the farmland in the family.
By the say, if you haven't done so already, you will need to file an Affidavit - Death of Joint Tenant if your grandfather is still on title to the land as a joint tenant. Furthermore, your grandmother should be holding the property in a living trust.