James, the answer to your question is as low as safely possible. High altitude cerebral edema (HACE) is the end-stage of AMS (conversely AMS can be thought of as the mild form of HACE). When you have HACE, your brain swells and stops working properly.
HACE symptoms include a number of signs of mental functions failing: confusion, fatigue and weird behaviour. But the most reliable one is gait ataxia, and you can test it by walking heel to toe along a straight line on the ground. Healthy people can pass this test easily; anyone who has difficulty balancing while they do it is showing signs of HACE.
HACE is extremely serious, and you may only have a few hours to help someone with HACE. The main treatment for this is descent, but a person experiencing these symptoms will need significant help. Dexamethasone is one drug that can be used to relieve symptoms, but it is just a temporary bridge to give more time for descent.