Percocet usage and driving?

My MIL is 87 and has been taking opiod pain medication for 5+ years, probably closer to 10 years because of escalating spinal scoliosis. Her medication contains 10mg of Percocet 4x per day. She also takes a muscle relaxant in the evening. She lives alone and still drives. How much medication is unsafe to drive or live alone? What are some questions we can ask her Dr.?