Hi Sylver, my daughter is 16 too and relaxed. I don't really know what to say, but that I'm so sorry this happened. It sounds like your dh had nothing to do with it, which is a huge relief! If my daughter went behind my back and relaxed I'd be upset, but if she conspired with someone else to do it, I would be especially heated, it's just so hurtful.
But Sylver, it sounds like your dd is a doll, with a sweet spirit. If in most areas she is a sweetheart and obedient and does her homework and doesn't stay out past curfew and speaks to you in a respectful manner, I would still punish her in some way but I would not be too harsh. This is a turning point in your relationship with her, I don't think you will look back on it and laugh, but you and her will look back and see how it changed both of you. I would share all of my advice with her and help her all I could. To be a teenager and have a mother that looks like you must be very hard, NOT SAYING WHAT SHE DID WAS OK THOUGH, I just think there is a beautiful lesson hidden in your turmoil.