I’m not a rav (far from it), but for the sake of a halacha discussion, here are a few points an thoughts:
First of all, even in a case where the mazik is completely responsible, e.g. someone deliberately kicked your car for no reason, the liability is only for the depreciation in value. Unlike secular law and insurance, the liability isn’t what it would cost to return your car to its previous condition, but only what its drop in value is. An already old and battered car likely has a minimal depreciation from a single additional dent.
Next, I think this question should’ve been asked to a rav and/or posek, because while this wasn’t your intention at all, it may be loshon hora about Chaveirim on this public site.
Third, I do think that the fact that it was Chaveirim is very much a factor – while a random person who comes to help you may or may not be competent, it’s reasonable to assume that a member of a group that specializes in this type of help will be competent, removing the “Why did you trust him?” question from the equation.
Fourth, my family members have been the recipients of Chaveirim’s incredible chessed multiple times. The fact that total strangers are willing to come out to you, literally get their hands dirty, and help with flat tires, jump starts, lockouts and other issues at any time day or night, all the while refusing any type of pay or even a tip, is just an unbelievable kindness. Often their help is for people who are desperate and have no other recourse to get their car back in action late at night or at other times where other help isn’t readily available or would be terribly expensive. These people are a G-dsend to the desperate and I’d strongly ask that anyone keep that in mind.
A word to the supervisor about better training for that individual – yes, phrased as kindly and appreciatively as possible. Being mochel the damage – also yes, whether or not you’re halachicly obligated to, it seems the mentschlich thing to do.