<human>: About 4 weeks ago,  the cats owner (we'll call her Rachel) realized her cat had fleas.  Rachel promised us she would immediately take care of the situation but she pretty much did absolutely nothing.

  She paid rent until the end of the month but stopped sleeping here about 2 weeks ago and started sleeping at her new place.  However, she left all of her shit, **including the flea ridden cat**, here in the living room.

Now,  not only does the house have fleas but she is maintaining that **"This is your guys fault too! You knew the risks of living with a cat!"**.

I should note here that when she moved in, she promised no less than half a dozen times that she would dust, sweep and clean up after her cat "AT LEAST once a week".

 As you can probably guess in addition to using the living room as her personal closet, she almost NEVER cleaned and would leave the litter box go until the whole house smelled like cat shit and one of us would have to ask her 3 times to clean it.

With that in mind,  none of us trusted her to properly address what has become a house-wide flea infestation so my girlfriend and I have been spending the day shampooing the carpets and furniture and spraying this god awful toxic flea killer.  

This is really just the latest incident in what has been a nightmare living with her.  Words do not describe the anger I am feeling right now.

Tomorrow she'll be coming to pick up the rest of her belongings and she'll be asking for me to return her $400 security deposit.

How do I handle this?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Shit head roommate's cat infected the house with fleas,  what should I do/how much, if any, of her security deposit should I withhold?