Please explain whos at fault?

my wife was recently driving over a blind summit on a wet day doing 30mph as we came over the summit a learner driver had stopped because a lamb had escaped from a field another driver had stopped behind them and this gave her no time to brake our tyres locked and we skidded into the driver in front the learner then drove away we have accepted that this is probably going to cost us but i dont think she should be held responsible any help on this would be appreciated