FG1

If he has crossed the broken white lines he has carried out a manouver.
Obviously, if there has been a collision, he has carried out the manouver whilst it has been unsafe to do so.
The very fact that you did not alter your course and he crossed the broken lines should eliminate any reasonable doubt as to the cause of the accident.
No matter what the road signs, even a green light, the direction is always followed by "when safe to do so".
Dig your heels in on this one.