Auto Insurance and who is at fault?

Hi All, I got questions for anyone or experts in auto insurance. My brother was involved in an accident a week ago. It was snowing pretty bad and my brother hit a lady in front of him who was trying to avoid another collision in front of her. She slammed the brakes and she was able to avoid the collision and my brother was not so lucky so he end up tapping her. She didnt have any damage on her car but my brother was left with cracked bumper. My brother reported to her insurance company since she suggested to call her insurance if there is any damage. Her insurance adjuster left a voicemail and said to fax the estimate of the damage. At this point, we were glad as we assumed she would handle the responsibility of this accident. We were gonna fax her the estimate today until we received a phone call from her stating that we should claim to our own insurance. Basically, she was telling us that it is not her client's fault that her client was only responsible for her own vehicle when she was trying to avoid the collision in front of her. In other words, its not her client's fault. So who is at fault then? what did she want to have the estimate in the first place? Can anyone help? Thank you.