Why didn't she... (SPOILERS)
Bash his head in with the crossbow as soon as the boyfriend has him in a headlock? Never mind trying to load the bow, if she whacked him she could have knocked him out cold.
Take both knives when she stabbed him in the bathtub? Deprive him of both weapons. She could have also just then stabbed him again when he was reeling from the leg wound.
Gather all the knives from the kitchen and the corkscrew and anything else pointy and just fling them like darts at the killer when he came into the house?
Bash his head in with the hammer when she had his arms jammed in the window? She got a good arm hit in, but could have done more.
I know there wouldn't have been a movie if she'd done this, but seriously. There were moments here when she could have gotten the upper hand so easily.