You have him returning and marrying, therefore some of the information on the below has errors as according to the below info he died 1915. Maybe the part about informing his daughter in law is a error as well??
From the link you provided
quote" Photocopied ts transcription (5pp) of extracts from letters written home by Private P H Allen during his service with the 10th Battalion Alberta Regiment, Canadian Expeditionary Force (CEF) on the Western Front, May 1915, containing graphic descriptions of his involvement in the Second Battle of Ypres, just before his death in action on 22 May 1915; together with a photocopied ms letter (1p) from J W Allen informing his daughter-in-law that her husband, Captain William Ruskin Allen, has been reported missing in action and advising her to "keep a stiff upper lip", plus a photograph of Captain Allen in a German prisoner of war camp."