What is the ruling on seeking treatment and spending the night in the shrines of saints and righteous people to ask for healing from illnesses, and how can those who do so be convinced that healing is solely in the hand of Allah?

Asking of other than Allah and requesting relief from distress or healing from illness from them is major shirk (polytheism), as in the words of Allah Almighty: "And do not invoke besides Allah that which neither benefits you nor harms you, for if you did, then indeed you would be among the wrongdoers." (Quran 10:106). And what happens to them sometimes in terms of extraordinary feats or healing of the sick is from Satan, a trial for the weak in faith. You must advise them to seek help from Allah and explain the danger of their deviation by presenting evidence from the Qur'an and Sunnah that harm and benefit are in the hand of Allah alone.