You had no idea, but Sarah more than fantasized about growth. She was obsessed with it. She virtually ate, slept, and breathed this one seemingly impossible desire to grow. Everything she came across that carried even the faintest hint that it could make her grow or grant her the power to grow herself and others, she tried it. And yet, she was still 5'5". She had vowed to do anything or pay any price if it would just allow her to make her obsession reality.
One day, after classes while you were with her, she came across a particular something on the internet. She went wide-eyed and you knew she came across something good, but before you could look over at her laptop screen, she closed it with almost a slam and ran off to her dorm. Before she did though, you caught her saying, "This could be it! But it's so expensive... Somehow..."
You had suspicions as to what she was referring to. After all, her odd questions seemed to hint at a growth fantasy of her own...or perhaps something far stronger.
Sarah had found what was being regarded as a breakthrough in science. An experimental serum that could induce non-cancerous growth in the subject. Granted, it was still experimental: it hadn't yet been tested on humans to see its effects, but they were unfortunately already closed and underway. A few sparks of hope remained, however...
Supposedly, a few samples of the serum had been...misplaced. Regardless of how, there was an individual willing to sell at least two samples of the serum. The catch was that he wasn't willing to part with them cheaply. Sarah had some money not tied up with tuition and living expenses she had dedicated to her desire, but not quite enough to pay the individual. She debated with herself overnight as to whether it was worth it or not to go all in on this, for it could be an elaborate scam, but the debate could only ever go one way.
So Sarah settled on three options as to how to get her hands on some of that serum and hope beyond hope that it would be worth what she was going to do. The first option was rather simple. She would attempt to take on as many jobs as she could and do whatever it took to raise enough money to pay for it. But it would take time, which could lead to unexpected expenses or the individual disappearing or the price going up. The second option was faster; have someone go in on this with her and share the reward. This way she wouldn't have to give up every last cent not tied up in anything else, and she could get the serum much faster. She suspected that you would help her as you were her close friend. Given your growth fetish, you might find it hard to turn down the offer... But she had no knowledge yet of it.
Finally, and perhaps the most dangerous and risky, was to steal. Either steal money from others on campus or try and track down where the serum was kept and steal it. It was quite expensive, and Sarah had vowed to do anything to grow. It might be easier if she had an accomplice, and she thought about you, but she didn't know if you would be willing to commit a crime for her. She herself questioned whether she was willing to, but it was covered under 'do anything', and this could be a once in a lifetime opportunity to have her dreams finally realized.
Some time later, you found her choice.