You get similar things with private housing - sell a plot and retain a "ransom strip" that the new owner has to pay to buy/cross.

When the Church of Scotland sold the local manse (that's what we call the minister's house, if it's different down there), they had a clause that said they would get £30k for each plot sold at any time in the future.

If it is in the titles I don't suppose there is a great deal you can do except pay up. See a lawyer with the titles to her house - his bill shouldn't be for more more than an hour's work to say if she can get around it or not.