agree with earlier poster, she is responsible for first impact. If there are injuries, then insurer will blame first impact for some of the injury. If your son was injured, I would guess that his claim, if there is one, will be shared on an equal basis between insurers of your gf and insurer of car that rear ended her. There will also be a discount sought in respect of her own injury arising out of the first impact.
Likewise if truck driver is injured, then his claim will also be dealt with equally between your insurers and insurers of teh car that rear ended gf.
There is no point in saying that all injury was caused by second impact. it will not be possible to prove that and in my view, a judge will not accept it either.
With regard to damage, you could get your own assessor to examine and report on the car. He could perhaps comment on the extent of damage to front and back. however, the rear ender must pay for all rear damage and mught make a contribution towards the front damage. Again, I suppose best you could hope for is a 50% contribution towards front damage. That being the case, you should get about €1000 for car.