Siegfried Sassoon famously befriended another World War i poet - Wilfred Owen. It was in Craiglockhart Military Hospital in Scotland, during August 1917.
Sassoon was there because he had written "Finished with the war" (linked below) and - rather than court-martial him, the military tribunal decided to send him to hospital and treat him for shell-shock (rather as if opposing the war was itself a sign of mental instability!). Owen was in fact suffering from shell-shock.
Dr. William H. Rivers treated Sassoon, and also forms the fictional protagonist of Pat Barker's novel "Regeneration" which imagines this meeting between the poets and their treatment from Rivers.
Both of the men eventually went back to the Front - Owen was killed just before the end of the war in 1918; Sassoon was wounded but survived and lived for many years, writing many works of literature.