Kids want to try stuff. My Mum gave us 'alcohol' once a year at new year when we were 10.. basically it was a soft drink with a smitter of something.. we thought we were special.. she also allowed us to taste whisky or sherry - I mean a tiny taste - (my parents drank this) when we were young.. 12 or over.. if we asked.. before that we never asked..
What her motives were was to 1) let us ask and she knew what we were doing 2) to put us off alcohol at an early age 3) know that we could always ask in a safe place (i.e. at home) and not outside being lured into trouble by others..
My own kids I did the same.. they did discover WKD by age 14 but it was literally 1 bottle each on our holidays or at a birthday or new year... and i've done the same with them as my Mother did.. let them ask & try...
But, in your situation, if my kid was 12 and her pal came over I'd say to the pal.. "sorry, you are too young and I am responsible for you tonight" - put the WKD aside and give it back when she was going home.. explain to my daughter the same.. too young.. and your friends shouldn't be taking alcohol to other peoples houses...
Don't be mad or fuming at them - they were actually very honest with you - your dd asked first... they didn't hide anything... which tells me someone is doing something right.. and they both sound like decent kids.. no need to kick the friend out or contact authorities - they are honest, curious kids and now you have a heads up that your dd is asking questions. you are now prepared for the next stage in her life and that's a good thing, you can guide her, not scold..
p.s. I am Scottish so our 'culture' is very pub/drinking orientated BUT, like I've said, good parents teach us not to be daft when we are young but let us ask & try in very small doses with parental supervision! I never started drinking till I was 23.. neither of my kids drink at all, except at new year and they are over the legal age now.. must be doing something right!!!! Good Luck