Herbert is listed as being killed in action on 25th May 1915. We have not been able to find any war diaries in Canadian or UK archives for May 1915 but we do know from the above history that they were part of "Seeley's Detachment". The following is taken from the website of the Royal Montreal Regiment (full article here)

"The final Canadian actions at Festubert were fought by “Seely’s Detachment”, which relieved the 2nd Brigade on 24 May. The detachment consisted of the headquarters and dismounted cavalry units of the Canadian Cavalry Brigade. Following the carnage of the 2nd Battle of Ypres, and in response to the desperate need for infantry, the cavalrymen volunteered to serve in France, and moved from their training camps in the United Kingdom on 4 May, numbering about 1,500 men, leaving behind their horses with British Yeomanry units. One squadron of Royal Canadian Dragoons received a day of instruction, otherwise, the 9 squadrons of dismounted troopers had no experience in trench routine or fighting. They went into the line opposite the also newly arrived 91st Reserve Regiment of the 2nd Guard Reserve Division, and on 25 May were ordered to cooperate with the 47th Division’s attack that evening.

The British attacked at 6:30 p.m. on 25 May north of the Givenchy-Chapelle St. Roch road and two battalions advanced 400 yards to take the German forward and support trenches on a 1,000 yard front, suffering 980 casualties in the process. Lord Strathcona’s Horse contributed a bombing group at 9:00 p.m. which worked north from K.5, bringing with them 200 gas bombs – as it turned out, the first authorized use of gas in the history of the British Expeditionary Force. They were assisted by bayonet parties and the Strathconas reported the South Breastwork clear from K.5 to L.8, a point 300 yards northeast, shortly after midnight. The 2nd Brigade sent work parties forward to consolidate but L.8 was found to be occupied by the enemy. The Strathconas had been as confused by the wretched maps as everyone else, and had occupied ground farther west than they believed. The 3rd Brigade, which relieved Seely’s Detachment on 27 May, was given the task of securing L.8 and link the Canadian line with the 47th Division south of K.5"

Further, Herbert's obituary on the King Edward VI school website states that he was shot in the head and died instantly while taking part in a bayonet attack.