What is the ruling regarding a man who does not fully grant his first wife her rights after marrying a second wife, when more than 15 years have passed since then?

A husband must be just between his two wives to the best of his ability. It has been authentically reported that the Prophet, peace and blessings be upon him, said: "Whoever has two wives and inclines to one of them more than the other, will come on the Day of Resurrection with one of his sides drooping."