First off you need to figure out what is causing the decline. Without a good idea of why it's declining then you have little chance of arresting the decline. I think trees like this are a great learning experience because you have a client willing to try and save the tree. Pay very close attention to what the tree tells you as you piece out the dead and pay particular attention to the transition from dead to live wood. If you see signs of boring insects then try to cut most of those out.
After you complete this phase of the job, pay attention to the tree in the future and see how it reacts to your pruning. Try to figure out what caused the decline and whether or not you arrested it. If the tree dies anyway then you will already have an idea why. Carry this knowledge forward when climbing other pines and look for similar signs, then follow up to see if those pines decline as well. After 20-30 years you will have a few more answers, but not all of them.