advice please

My ex-roommate, Kat, told our landlady, Simone, that we were moving out of our apartment a month before our lease ended, that we were leaving at the start of June rather than the start of July. Our landlady couldn't get anyone in to rent for June so it's standing empty. Simone did not make us pay rent for June but did keep half the security deposit. Since Kat hadn't paid into the security deposit, I lost $400 out of the deal.

Kat wants me to reimburse her for the last of the bills, which comes out to $80. I responded that she owed me half of what Simone kept from me. I think Kat owes me $200. If you take out the bills, then she owes me $120.

After I replied with that, Kat told me that Simone had kept the pet deposit, which Kat had paid since the dog is hers. That's $300. However, Kat changes facts to make her argument so don't necessarily believe that the landlady kept the pet deposit. I don't necessarily disbelieve it because when I told Simone that it was important to Kat to get it back, she just gave me a look and didn't comment.

So I sort of want to contact our landlady and ask if she kept the pet deposit. On our walk-through of the apartment - Kat was out of town by then - Simone and I agreed that it would be fair if she kept half the security deposit. Nothing was said of the pet deposit.

Any suggestions on what I should do?