Answer: The symptoms you describe could be caused by many different factors including severe winter damage, moisture stress, pest or disease, root damage, aftermath of being badly pruned, or due to a change in drainage patterns for example.
If the stones have plastic underneath them, the trees may be suffering from lack of water at the roots. If there was an herbicide applied to prevent weeds in the stones, that might have damaged the tree roots.
It is difficult to make this kind of diagnosis long distance, so if your neighbor is concerned about her trees she might consult on site with a professionally trained and certified arborist with experience in ornamental trees. Or, she might consult with your local county cooperative extension.
I'm sorry I can't be more specific long distance.
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