I think it's fairly arrogant to take it upon oneself to determine whether or not someone is cut out for a particular field of study. I know I'd feel incredibly uncomfortable making a statement like that when my own personal feelings on the subject could possibly stop someone from going on to do great things. Perhaps start a conversation about other fields the person in question might think more about, but unless they're a total loser or you despise them, if you don't think you can write a letter recommending them for something in particular, then just write a letter about the qualities you do admire in the person. The school (or wherever they're applying) has enough other information, presumably, to determine whether or not the applicant is suitable for what they want to do where you don't necessarily have to confront that issue in what is presumably going to be a few paragraphs.
Write her the best letter you can.