This damage looks similar to what I did to my Fatboy. I found out the hard way that it doesn't take much to dent the fender (I'm at work now, but will try to remember to post a picture of the damaged fender). I was moving my bike in the yard and backed into a sapling (about a couple inches in diameter and dented my fender). I was able to get a factory painted fender and it matched perfectly. As I remember the cost was about $250 for replacement fender, then cost to have it installed.
I think the positive approach that has been suggested would be your best bet since it wasn't noticed when you picked up bike. My guess is that you are meticulous about maintaining (mechanically, and cleaning) your bikes, and you can bring that point up to them as this would have been something you would have noticed or known about if it happened while in your possession. Also remind them of the good business relationship that the two of you share and you would hate for this accident to change the way either of you feel. Hopefully they will do the right thing and realize that in the interest of good customer relations, and future purchases you may make that it would be in there best interest to take care of this.
Hope it works out for you.
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Ride Safe,
Fired00d
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