Is buying computers in installments from a friend, after taking the necessary money for that from another friend who deals in installments, considered usury (riba)? And what is the ruling on someone who bought these devices and was unable to pay their price?

Murabaha sale is permissible if the seller first buys the commodity and then sells it to the buyer in installments with a known profit margin. There is no objection to the buyer acting as an agent for the seller in purchasing the commodity. However, if the matter is merely giving money on the condition of its return with an increase, then it is a forbidden usurious loan (riba), based on the Almighty's saying: (And if you repent, you may have your principal - [thus] you do not wrong and you are not wronged).