Massachusetts General Law, Chapter 278, Section 8(a): In the prosecution of a person who is an occupant of a dwelling charged with killing or injuring one who was unlawfully in said dwelling, it shall be a defense that the occupant was in his dwelling at the time of the offense and that he acted in the reasonable belief that the person unlawfully in said dwelling was about to inflict great bodily injury or death upon said occupant or upon another person lawfully in said dwelling, and that said occupant used reasonable means to defend himself or such other person lawfully in said dwelling. There shall be no duty on said occupant to retreat from such person unlawfully in said dwelling.
This is the clarified Massachusettes law that covers a duty to retreat, which has been removed. In essence, no, there is no longer a duty to retreat when one is is reasonable fear of "death or grave bodily harm". This isn't taken to mean that one can just smoke someone for being in the house, but one must be able to articulate why a reasonable person, knowing what you knew at the time, would have acted in the same manner.
The nuts and bolts are this: If you are in immediate and otherwise unavoidable danger of death or grave bodily harm, then you (or your spouse) has the right to use force, up to and including lethal force, to stop that threat especially inside your own home.
As long as the firearm used defensively is legally owned by you or your spouse, then there is no legal issue in Mass that I could find. Even if there was, an attorney can argue "competing harms" insofar as it is unreasonable to NOT use an available firearm owned by a spouse in a situation where a lethal threat is faced. The "greater harm" is death or grave bodily harm caused by the presence of a violent attacker than grabbing a firearm that she may not be permitted to carry outside the home.
So, the reality is, in short, inside your home...she's good as long as the shoot is righteous otherwise.
"That man broke through my door, he appeared to be armed with a gun/knife/greater size and strength and he said he was going to rape/kill me. The gun/knife is laying on the floor by his right hand. I was forced to fire to stop him. I want to give you my full cooperation, but I am really shaken up right now (and you/she will be), so I want to wait until my attorney is here before I make any more statements".
Do NOT go with "I am having chest pains" or any other nonsense such as that. A simple statement of "I am really shaken up" is HONEST, and understandable. Do NOT get into specifics of how many rounds you/she may have fired...this can get you into a trick bag if you believe you only fired twice, but it turns out in the adrenaline dump that you fired a whole magazine/cylinder full into the attacker.
"I am not sure how many rounds I fired, I stopped firing when he stopped his attack".
Be safe.