What is the ruling on sleeping next to a friend under one blanket for warmth, whether it is the only available blanket, or it is a blanket that does not provide warmth unless there is another blanket?

It is not permissible for two adults to sleep under one blanket. This is indicated by the Prophet's (peace be upon him) command to separate children in their beds when they reach ten years of age. It is, therefore, even more imperative for adults to be separated. Adults should be separated into two beds and two blankets. If this is not possible, then each should use a thick covering. Al-Hafiz Ibn Hajar emphasized the condition that a group should not gather under one blanket.