What did the botany student eat, if not previously living beings? Dirt?

No, that's just it. She ate previously living things, just like absolutely everything alive does (except for those that eat currently living things). She just didn't want to think about the fact that they'd once been alive, and was very uncomfortable discussing it.
(Actually I'm not sure if it's the "alive" bit that bothered her. But she was very squicked by her friend pointing out the plant-anatomy of the vegetables they were buying...)

The only problem was finding honest language that was in or near his vocabulary.

Well, there's that of course.