Advice needed on large mock cherry tree looking very sickly?

My neighbours have a very large mock cherry in their garden right on the boundary with our garden. About 3 years ago the previous owner paved over the whole of their garden which is only tiny and erected a fence right against the tree. Since then all the shrubs on our side of the fence have gradually faded and died apart from the Hawthorn hedge which although now only has leaves on our side. Now I notice that the tree is showing signs of dying on one side despite it having had more blossom on it this year than I have ever seen on any cherry. I have been watering my side of the garden in a desperate attempt to keep my shrubbery going, could our very limey water be the cause of the cherry looking so sick?