Worth a claim or not?

ok this morning i was approaching a set of traffic lights and had to brake quite harshly, i looked in my mirror and saw the car had plenty of stopping distance, but about 4 seconds later the car behind shunted me. i have been down the hospital all afternoon with my girlfriend who was diagnosed with whiplash injuries. now my questions are, should she claim? will she win? approx amount? what happens if she looses the claim? cheers in advance. i know you lot can help me out ;)