Is it permissible to sell a house to a buyer who will take out an interest-bearing loan from a usurious bank to pay the price, and is that considered cooperation in sin and aggression?

It is permissible for your friend to sell his house to someone who will buy it with an interest-based loan, because the prohibition of usury applies to the borrower. An interest-based loan — though forbidden — confers ownership of the money upon the borrower in some schools of thought. Nevertheless, your friend should advise the buyer about the impermissibility of usury. The buyer can look for a permissible alternative, such as purchasing the house through Murabaha.