You decide she's been a competitor for too long. You want to strip away her intelligence, but it might be best to take this piecemeal. You don't want to cause her willpower to break the spell.
"You are of average intelligence," you state.
She seems a little shocked when she says,"I... I'm not..." Even so, Harvard now no longer ever accepted her. She still got into Yale, but you remember how upset she was about it, and how bitter she was with you.
"You are of average intelligence," you repeat.
"I... I am normal," she sighs. Mei still wasn't dumb, but she just wasn't special. She got into average schools, but no where particularly special. You helped where you could, and she is no longer bitter with you. In fact she is grateful, feeling that she got into those schools because of you.
"Actually, you are quite dumb," you state.
"I... I not smart," Mei says. She seems on the verge of tears at this point. She managed to get into community college, but that's about it. She was helped by you, of course, but it was barely enough to pass.
"It's probably because of how much you care about your looks," you decide to add.
"I care about my looks." Her outfit becomes a whole lot more revealing. Her midriff becomes exposed and her skirt turns into shorts that aren't full on short shorts, but are still pretty short.
"It probably helps you get those guys,"
"Me like sex," she says. At this point she lacks the will power to resist and you two have become so different you're no longer friends. Occasionally you tutor her, but she gets most of her help from asking guys to do it for her.
You've successfully ruined your closest friendship and someone's life.