My partner's had a car crash, who's fault is it?

Hi my partner had a car crash at the weekend. He was driving towards a T junction when the lights went amber as he was apporching, approx a car lenghs away from the lights he broke but the surface of the road is very slippy when wet (which is was) he began to skid but hit a car turning into his path. They say it was his fault but I don't think it is. HELP!!!