If you know they’re already having sex, and you confirmed her parents are OK with her sleeping over, I don’t understand why you’re asking this question.
While I agree with the general sentiment, the vinegar won’t be released and diluted until later in the load, so it sits in its full 5% acetic acid glory for quite a while.
If you know they’re already having sex, and you confirmed her parents are OK with her sleeping over, I don’t understand why you’re asking this question.