If the bad guy was shot in the back, it might be awkward unless he was between Howard and the girl, and facing the girl.
Looks like Howard did a good job sneaking up on the perp. No sane person wants to turn this kind of situation into a “fair” fight. Drop the bastard before he can drop the girl or the CCW holder.
Shot in the back really has no legal significance outside western movies. There are a million ways it is ok to shoot ‘em in the back. One is by being an armed man in a store, clearly out to hurt someone. Any angle you can hit him from is fine.
A cop normally has a duty to verbally challenge, but a 71 year old man would fall into significant disadvantage by announcing his presence to a fit 19 year old.
If it was clear to everyone in the store that the guy was there on a mission of evil, any shot is in bounds legally.