Still its a good thing they did.
A couple of points to note.
Firstly, it is probable that Drummer Lee Rigby remained 'alive' and possibly even 'aware' for several minutes after his beheading, if being run down by a car did not kill him. Though cut off from oxygen and blood, the brain remains 'alive' for a short period of time. I have read of 'recipients' of justice from France's 'National Razor', making facial expressions and their eyes even following objects, for a short period of time after its use; rare, though it has occurred. It is possible, even if his head were completely severed, for him to hear sounds, even if he appeared unconcious, prior to death. As he no longer had a trunk to allow breathing, he would be unable to speak or make a sound. The prayers of these two women may have been a comfort to him, and most certainly could have been for those near by.
Secondly, as a Christian, if Mr. Rigby were not a Believer, his hearing their prayers, may have afforded him the opportunity to accept Jesus and be saved. We know that the repentent robber on the cross obviously had no time to perform good works, yet Jesus told him that, "...today you shall be with Me in Paradise." (Luke 23:40-43) On that basis, I believe that there was an opportunity for his redemption, if he was not saved.
Thirdly, I have seen some amazing things occur involving healing among family and friends, things that doctors can only explain as 'flukes', or cannot explain, but yet, deny even the possibility that God could have performed a miracle. Remember, in Canada, the vast majority do not believe in God and, in fact, many think of themselves as gods!
A gentleman at a church that I used to attend, witnessed a man come back to life at a prayer service, after having been dead for 15 minutes. I have found no fault in anything else that he has ever said, so I cannot believe that he would lie about this.
I am currently assisting a Pastor friend, originally from Liberia, to set up a church in a lower class neighbourhood in Calgary, an isolated community of 6,000 people, with no churches for two miles. Some of the stories he has shared of what God has done, just amaze me! In Africa, they see and acknowledge many more miracles than we do.