How do you know?

I would never advise someone to say or do this without knowing a lot more facts.

For all you know, one or more of his insurance policies may provide for the defense of the claim. Why on earth would he not then consult a lawyer?

There's a lot more that needs to be learned and a starting point is likely with his homeowners and other liability coverages. As noted above, some of those may have strict time deadlines. If OP doesn't notify and is later found liable, he may have zero coverage from those policies.

I don't think that those who blithely advise OP to treat this as a debt collection (which it does not appear to be) or to deny that they have coverage and ignore it, are particularly helpful or even well-intentioned.