I was about to post about this myself.. thanks.
Though no weapons (other than fists and feet) were used, this apparently resulted in serious bodily injury (I fear the siezure indicated that the victim sustained brain injury). Legally, in many/most states, imminent threat of serious bodily injury or death justifies use of deadly force. However, if one were to intervene using a firearm, thus preventing this serious injury from ever occurring, I can well see the third-party defender acting on behalf of the victim who was unable to defend herself, being charged, prosecuted and possibly being convicted of homicide or attempted homicide. I don't think I could stand by and left a tragedy like this happen. What would then happen to me? I fear that I already know the answer, it is disturbing to contemplate...
I am a smallish male in my 50's, not a fighter, and perhaps I could overcome two unarmed females attacking another, if I really let-loose, so maybe that would be a much safer tactic, legally speaking... Three or more, I don't think I would be successful without employing a weapon..