Why do you feel like you would be betraying her? Especially if you are considering the possibility of underage drinking...she is putting herself and her parents at risk. There should be no mixed feelings here, IMO.
Since you are that close to her you should be able to discern from what she is telling you if this is just a little get together among girlfriends or if this is truly an all out party in which the chance of underage drinking is very good.
I think that your loyalty should not lie with her, but really with her parents. They trusted you to keep an eye on their home and you agreed. Parent to parent, you owe it to them to report anything that may place their daughter or property at risk. It really is for everyone's benefit, even if the 18 year old gets upset. This is a safety issue, and the 18 year old, as great as she seems, is not mature enough to fully grasp all the risks of hosting a party behind her parents' back.
In an auntie-knows-best sort of way, I would have a serious talk with your friend. Ask her directly the details about who will be invited and if there will be alcohol or drugs. Also, inform her that since she is 18 (and not a minor) she will be held acountable for her guests. And, since she is choosing to host this event at her parents' home, then her parents will also be held liable. This could mean lawsuits that could cost her parents thousands if a guest is harmed on the property, depending on the situation.
Finally, I would let her know that you will be informing her parents about this, to make sure that they are aware (and keep in mind it is possible they already know about this, or maybe allowed a get together but not necessarily a party). It is fine if she feels "betrayed." Explain to her that this is about safety and not about ratting out a friend, but expect that she will not be able to understand this so she may be angry at you. And that's OK -- you are thinking like a parent now, not like a bestie.