How to prevent Brexit from damaging the EU

What should the EU’s priorities be in negotiations with the UK over Brexit? Paul De Grauwe writes that the EU should offer the UK two options: either following a Norwegian style model or leaving entirely and negotiating a free trade agreement in the same manner as other nations such as the United States and Canada. He argues that offering any concessions outside of these two options would risk fatally weakening the EU and should be avoided at all costs