My gf was in a car accident, the lady that hit her ran a stop sign in a parking lot and now their insurance is saying the person that hit her isn't at fault. Any advice on what to do?

My girlfriend was in a car accident last week, she has only been driving for a year and this was her first accident. The accident happened in a new Walmart's parking lot, and we 100% believe that the other driver is at fault. I wasn't there and there were no witnesses, but the other driver had a passenger. This was her first accident like I said and so she didn't know what to do other than exchange information; they didn't take pictures, but they sent each other pictures the next morning. My girlfriend got a letter yesterday from the other persons insurance saying that their client isn't at fault even though they haven't communicated with my gf's insurance company at all.

Here's what happened: Girlfriend was at a stop sign to turn left to head to the signal so she could exit the parking lot, this is a 3 way stop sign intersection. The person that hit her car was at a different stop sign in front of her to turn into the 3 way stop's stop sign, she didn't stop at the second stop sign and that's when she hit my girlfriend's car.

The intersection looked like this: http://imgur.com/Ko4KOTk

What can my girlfriend do to dispute the other persons insurance since she is obviously lying and ran a stop sign?