I've been following this case a bit.
The problem with it as a self-defense case is that it wasn't legitimate self-defense. Ms. Alexander had left her husband and taken out a restraining order against him. She went back to the house where they had lived at a time when she thought he wouldn't be there in order to retrieve clothing. He was there, they argued, and she went out of the house to her car, got her gun, and then went back inside and fired a shot. She said it was a warning shot; her estranged husband said she was aiming the gun at him.
She said she was afraid for her life, and claimed self-defense under Florida's stand-your-ground law. This was rejected by the court, on the grounds that she could have left the property rather than returning with a gun.
She was offered a plea bargain that would have meant a 3-year sentence, and she rejected that.
She was convicted of assault with a deadly weapon; under FL law, using a firearm in the commission of a felony mandates a minimum sentence of 20 years, which is what she received.
There's really nothing to see here, Second-Amendment-wise. Is the 20-year sentence wrongly applied in this case? Perhaps, but with mandatory sentencing laws, the judge has no discretion.
See this story at HuffPo for more details.