nothing, you wont win, wipe your mouth and move on
it happens every day all across the country, we've all been there, wanting to smack the "friend" right between the eyes, i usually find this "friend" knocks them on something mechanical, maybe he says clutch bites too high or theres a noise that nobody else can hear, the customers happy but the friend isn't, forget em move on, you will not win, and you wont hear from them again, because as you say, trusts friend more than "us".
What normally happens is here you get angry when theyve gone, and on wednesday you sell the car and take a part ex on which you make more than the original car, the sales done in 15 mins, and you think hey! why did i loose my head on sunday with them shits !