There is no right answer here
I will say this guy better be quick and hope the husband isn't a fast draw because his butt is grass if he is tanked at the scene before he turns his back and starts running. I don't see the husband getting charged in that scenario(unless he is in Hawaii and by protocol they arrest you as procedure even if it seems justifiable). Probably only off-duty LEOs couples in Hawaii will have firearms on them anyways.
**it is making me wonder about that agent just arrested @ mcdonalds in Hawaii because they arrest automatically after a homicide(justifiable or not)?!