I’m not justifying what they’ve done in anyway, it’s out of order and I’d be tamping....
But, if there’s no physical damage to the car as a result of their short (but fast) excursion, and you’d have been none the wiser if it wasn’t for the dash cam footage ...then no harm seems to have been done.
If you’ve not owned the car from new then any of the previous owners could have driven the car in a similar, or even less sympathetic, manner for a lot longer than the few minutes they were out in it.
I realise I’m stating the obvious, but thinking logically rather than emotionally is probably better for your blood pressure here.
I guess the question is ‘how much stress and potential grief do you want to expose yourself too?’
I’d send a polite email, explaining that you have been using them for years and you are really disappointed by what you saw on the dash cam footage after collecting the car. I’d say that I was happy to provide a copy of the footage, but that I’m certain you know what I’m referring to and ask politely for ‘an explanation and their thoughts...’ or words to that effect.
Hopefully they’ll realise you’re being reasonable and not threatening to post stuff all over the internet about them and common sense will prevail. I’d be bloody embarrassed if it was me and I was caught, I’d be apologetic and realistically I’d offer a refund as a gesture of goodwill.... in reality he has more to lose than you and you’ve done no wrong... and hopefully he should realise that.