If it was me, personally - given the state of the rest of the panel, and the damage which will come out with a few minutes of polishing, I would have suggested to him that it'll polish out, and that I'm happy to do that, and give his car a good clean as a measure of good will, explaining that "detailing" is a hobby of mine and the fruits of such would be clearly visible in my car anyway.
In any other situation, something £300 or less I would try and avoid insurance and offer to pay for it myself, providing he at least gets a quote from a place of my choice*.
Anything over, or if it was a particularly nice/new car that the owner is going to insist on it going back to the dealer for, I'd just hand over my insurance details - that's what it's there for.
*I had this a few years ago, after taking a friends E34 525 home to do a job on it, I went to leave for work the next morning after sleeping through my alarm. Jumping out the door probably 60 seconds after waking up, and straight into the car, in my still "comatose" state and being in a rush, I swing the wrong way out of my space and swipe the off-side of my neighbours car. They were out at the time, so I carried on to work.
Upon returning home, I went straight round there, explained what happened and told them that I would foot the bill seeing as I was only covered under works insurance to drive the car, I didn't want to get into the complication of why I needed to drive a car third party.
So they went away, got some quotes and came back to me with three bills ranging between £950 and £1800. Needless to say, I laughed and asked them if they'd mind me getting a quote from a bodyshop (a family friend) that I've used before. After demonstrating the work he had done for me before, they agreed and a day later he came round to look at the car. There and then he said I can do that for £200. Something so cheap was a bit off putting for the neighbour, but with the confidence I had in his work, we came to the agreement that if they weren't happy with the job, I would pay for it to be rectified at one of the places they'd had a quote from previously.
He came and took the car a few days later, had it for two days and it came back perfect. The moral of this (rather long and almost pointless) story is that many bodyshops will replace things that simply do not need replacing because they think in terms of how much they can get away with doing with the insurance assessor and not what is economical.