Police in college towns have their own ways of learning about the locations of parties where underaged drinking is likely to occur. It's not unusual for them to lay in wait for minors who straggle from the party.
A police officer has the right to stop people walking on the street and to ask questions or to ask to search them. Your daughter could not have just ignored the officer altogether. That said, she did not have to admit to any drinking, nor did she have to agree to follow the officer back to where the party was. She could have asked if she was free to leave, and if he said yes, to have gone at that point.
Underaged possession or consumption of alcohol is a strict liability offense. That is, they don't have to be drunk in order to be charged with a crime. They don't even need to have been drinking at all, if they are found at a party where they could have had alcohol. If they're found elsewhere, any amount of alcohol in their system can get them charged.
Can your daughter plead not guilty despite having admitted to drinking? Yes. She has the right to go to trial if she wants to take the case that far, and to do that she must plead not guilty, which is an option available to all criminal defendants, whether or not they actually committed a crime. Our constitution allows every criminal defendant who wants to to put the state to its proof and try to prove guilt beyond a reasonable doubt.
If your daughter does this, her attorney (she should have one if she expects to win a criminal trial) can ask for a hearing to challenge the stop and her statement. If she wins the hearing, the case can be dismissed. If she doesn't, she will likely end up with a criminal conviction on her record, because the breathalyzer results and her statement will come in and that doesn't give her a great chance of success at trial.
Typically, however, this case won't get to trial. On a first arrest of this sort for a minor defendant, your daughter will likely be steered into a diversion program. That's where she'd spend a period of time on supervision during which she'd have to stay out of further trouble with the law, refrain from drinking and drug use, perform some community service, pay some fines and take anti-alcohol classes. Upon completion of all of her conditions, all charges would be dismissed so that she could walk away without a criminal conviction.