Try to use logic.

If there is a way to break the spell with contradiction, maybe you could use logic to reason Melanie free of the magic.

"Alright, Melanie... let's experiment a little. You obviously have an attraction toward Amina here," you say, grabbing her chair so that you can face her as she lays on the bed, ogling her bedmate.

"Mmhmm... seems to be mutual," Melanie agrees.

"Tell me, are you normally into girls this way?"

"Well... not really. I mean, I've played around, sure. I'm in college after all. But, I prefer cock to pussy," Melanie says, "Then again... I've never come across such a delectable sample as this one here..." she snuggled up to the petite redhead and they both share a giggle.

"Alright, so you're not into girls, but you're into Amina. Can you tell me what makes her so attractive then?"

This question makes Melanie sit up and examine the object of her affection. "I don't know... I like her shape, slender and delicate. She has a cute face... soft skin..." Melanie brushes her hand over the aforementioned skin and eyes Amina with a warm smile.

"So, you would say the attraction is physical?" You continue. You're not sure where you're leading to, but it doesn't hurt to examine this situation scientifically.

Melanie pauses and thinks about it. "Well, I suppose so. I mean, I hardly know her that well. We just met. But there is some primal attraction that draws me to her." She almost leans in to give Amina another hungry kiss, but then she stops. She turns to you with a raised eyebrow, "What's going on here anyways? I thought I was conducting the survey..."

"I'm just trying to figure out how this all went down," you say. "I mean... do all your experiments end up like this?"

Melanie sits up and pulls up her bedsheet. "No... they don't..." she admits as things run through her head. "What did happen just now? Why do I feel this way about Amina? What is going on?"

It seems the student's analytical mind has taken control and she is now questioning her actions.