If he wants to continue to do his mowing he will have to get insurance as already stated.
Down here it is called public liability.
HE might consider this a "cheap" lesson as it is only a van window.
If it hit a young woman on the face enough to leave a scar you are talking thousands .
As for making a claim on the customers insurance, forget it.
As previously mentioned they will come after you for the money + costs and then could jack up the premium of his now lost customer
HE will also need to have a look at how he trims to avoid tossing up rocks and might even get a better guard for the trimmer.
Lots of after market big guards.
My commercial customers use a dedicated edge trimmer for this reason and because they give a cleaner sharper cut, but edge trimmers have a much better guard than a line trimmer turned sideways.