Thanks insom, you rule. I agree with all of this and it's the route I plan on taking. I especially like the idea of paying her mother and allowing her to decide how to administer it. She did do a nice job watching the dogs and I really do think she's a good kid. When talking about this last night, I mentioned to my wife exactly what you said, it probably started off as a few friends and then word got out and all of a sudden it's a bash not a get-together. Great advice! Thank you.
Last advice, if her mom looks really upset or angry, remind her that her daughter is a good kid, a good worker, and smart. She just made a mistake. I'm the oldest and so my parents went through all my shit for the first time. My friend had 3 older brothers so her parents were already experienced in the teenage ways. My parents would get really wrapped up and act like a nug of weed was the the end of the world. They started crying when they found out about the party. My friends parents reminded them that of my group of friends I was the most responsible and that I put school and work first unlike their daughter and it opened their eyes to the bigger picture. Sometimes parents can forget that their kid is actually a good kid when they fuck up.
UPDATE: So I talked to her mother and let her know what I found. She was great about it, she was disappointed and used the word "sad" to describe how she felt. She said that she would ask her her side of what happened and let her try and explain what happened. We both agreed that it was likely a situation where she had a couple friends over and it got out of hand. I just received a text message from the girl and she was EXTREMELY apologetic. It seems very sincere and it's pretty lengthy. Some quotes: "I don't know what came over me, I am ashamed". She says that she only had 5 friends over to celebrate one of their birthdays and "apparently I don't know my friends that well". She doesn't want us to pay her and wants to pay for any "damages". There aren't any damages, but now I'm wondering if she knows something I don't. She's going to come by with the key and I'll get a chance to ask her about what happened. She said she realizes she abused our trust and it makes her upset because she really likes our dogs and our daughter. Thanks for the help/advice everyone. Especially you insom, I essentially used your approach and I think it proved to be the right one.