I wrestled with this for just a little while but decided I'd take the shot for two reasons.

1 the deer were very likely doomed to a slow cruel death and the opportunity to prevent that and maybe save one should be taken

2 The fair chase issue is a bit cloudy here so I thought a while and found a perspective that makes me comfortable with the shot. No human intervention caused these deer to become vulnerable,no fence keeping them in,No vehicles, lights, electronics etc etc were used. The hunter took advantage of something that we all know occurs in nature (even if infrequently). If it's OK to take advantage of a mature buck letting down his guard in the rut (a more frequent but no less deadly situation for the buck), why not take advantage of this situation?