Before you decide which type of buckshot to use... reduced recoil or full power, or 12 gauge vs. 20 gauge, you should consider first if they can effectively wield the shotgun in close quarters against multiple opponents.
My wife, 5'2" can shoot a 12 ga. Mossberg 590 pump with 20 inch barrel accurately and quickly. No problem with recoil! No bruises or getting knocked on her butt and no getting the pump action in a bind from short stroking the action. She knows an aggressive forward leaning stance to shoot from.
However, in the close confines of the home, and in a "gun grab" situation, she just doesn't "wield" it confidently. In other words, it would be rather easy to disarm her. And that is a very legitimate concern everyone should have.
The good thing is that she realizes this and even though I have worked with her on long arm retention drills, she still does not feel confident with it as a primary home defense weapon.
Fortunately, we have some other options. She much prefers her pistol or the AR-15 collapsible stock carbine as a primary weapon and is much more secure with those in a potential "gun grab" situation. In my opinion that AR-15 carbine is a natural for women. It certainly is for my wife.
However, she has no problem using the 12 gauge shotgun as Ayoob suggests. Which is being used as an "artillery" piece from inside the ensconced/barricaded position inside the bedroom, and using it from behind cover of the bed.
When she's at home alone, she has at least 2 pistols within arms reach in the living room and usually has the AR-15 handy at the end of the sofa.
For self defense rounds, we use the Hornady Personal Defense .223 rounds in the AR-15.
While the Shotgun is an awesome defensive tool, you need to ensure that the person using it is confident and competent with it in all situations including CQB where a potential gun grab can occur. Especially if there are multiple intruders who could rush her at any time.
Just make sure it is an appropriate tool for your loved one to use before turning them loose with it. It does no good to be able to effectively neutralize one attacker only to be killed by his other two accomplices because your loved one couldn't effectively control it during a disarming attempt.