Good post, girlzRprettiest. Sorry to arrive late at this discussion.
Your friend may have been thinking that consent is not enough for ethical acceptability and that INFORMED consent is needed.
The trouble with this argument is that, assuming there is no risk of pregnancy or getting an STD, there is usually only one thing the girl would need to know - that some people may despise her for being involved in this activity. But,depending on her level of intelligence, the adult may be able to get her to understand that. Also, this 'slut shaming' tendency is not a given. It can and should be opposed.