Was it in someone else's property or on the public highway/pavement etc? If someone else's property then your son needs to notify them. If the highway then notify the council.
Trees or not ? Wayward branches are not trees. Adjuncts to a tree
A good case for Rumpole.
When the offender a tree really a tree and not just a hedge?
Did, in fact, the car drive into a hedge ? Allegedly, might not have really been a tree at all
What vehicle was this allegedly hedge assailant (cf. murderer) driving? What speed was the allegedly offender who precipitated the branch falling on him driving?
Summary: do not admit to controversial facts. When is a tree really a tree at all? Ask them for measurement of trunk dimensions etc. Make of tree? That should waste a bit of time.
We do not know even know what species (Latin name) of alegaedly offending tree it is? Bashed on the bonnet by a Macrocarpa?