Firstly we can all see the funny side of this, but it's more a joke between friends and not for customers, let alone their belongings. I clean cars myself and would never dream of doing that to someone else's property no matter the make or model or age. But reading through your local dealership has offered your a verbal apology, by no means can they control and outside company, you should shame the outside company.
I'd try IPA ( isopropyl alcohol ) you can get it in Boots or a Supermarket very cheap, it'll remove waxes etc so hopefully give your friend thorough clean

, wax won't shift easily so it sounds more like what it was, I'd be hesitant to clay the inside due to the super wet nature of claying. Depending on where you are in the country there are plenty of professional detailing companies that take pride in their work and could sort this for you I'm sure free of charge during a valet of your car.
Kind Regards
Willy ( not the one on your windscreen! )