Then he would be have already displayed some serious issues, and there really wouldn’t be any use covering for him. Plus, he isn’t your responsibility at that point, so I see no need to help him out. The only viable damage control would be in fact to say no and not involve yourself further.
This is presupposing that the reason you must say yes and the reason you want to say no are one and the same – seeing as we still need the latter to figure out what the heck’s going on.