The laws vary among the states, but there is a distinction between "rape" and "unlawful sexual intercourse". Since "your friend" is a minor as well, this will likely fall under the lesser offense of unlawful sexual intercourse (for both parties). However, the alcohol could complicate that and escalate it to a higher level, depending upon the circumstances.
As said, the 13 year old is incapable of giving consent, so it doesn't matter the least bit what she said or how willing she was.
Also, if the parents were aware the minors were consuming alcohol then we would have to consider whether WI law permits parents to do so within their own home. Probably the worst the parents could face is contributing to the delinquency of a minor, however there would have to be clear evidence that they were aware of the sexual activity in order to have any responsibility over it. It sounds like a situation in which the adults were present, however there wasn't a lot of supervision going on. That argument won't likely go far.