Let his insurance company do it
He needs to rell them who is responsible for Highways in your area (in ours, it's the county council) and send a report with the dimensions of the pothold - including depth, and some photos.
If the insurers make recovery, he'll probably still have to contact the council direct to recover his excess, but he should take advice from his insurane company as to when's the best time to do that.
No point in suing the council in the first instance - that's what he pays his insurance premium for, they'll do it.