My question is...do you believe your daughter? I only ask 'cause you said she claimed...and claimed is one of those words...
Gut, the weird thing is...if the kids were doing something sneaky, that they know they shouldn't be doing, wouldn't they have done everything in their power to HIDE the evidence? (e.g. thrown the bottle/caps away in someone else's trash can?). It's almost like someone WANTED you to find them... Is there any chance the bad friend planted this in the tent for you to find to cause trouble for your daughter?
If your daughter participated in the drinking and didn't want you to find out, she probably would've offered to take the tent down herself so that she could hide the evidence. So she could very well be telling the truth.
I would talk to your daughter again and tell her that you want to believe her, but this is a serious case that could have legal ramnifications for the whole family so she needs to be honest with you...and that even if you punish her, you still love her more than life itself. (I know that's corny but I also know that it's true for most parents.) Tell her that if need be, you will pay for the DNA to be analyzed on the bottle and that will reveal who sipped from the bottle. (Of course you don't have to do this...and she may be too savvy to already know this but...)
If after all is said and done you don't believe your daughter is being forthcoming, can you take her to a MADD meeting to listen to what can happen when underage (or any age!) people drink irresponsibly?
Regardless of whether the bad friend is responsible for this or not, I wouldn't allow that girl into my home again...and I would question why your daughter is hanging out with her after the harmful things she said about your Ex. Also, if there are any more get togethers, I would insist on knowing in advance who your daughter has invited so you can approve it before any invitations are extended.
Good luck, Gut!!!!