I've heard good things about Smultron.

BBEdit used to be the go-to app for text editing on Macs; I haven't heard much about it lately. (Oh, turns out TextWrangler is the free version of it.)

Personally, I use vim, which is included with MacOS, but runs inside a terminal. There is a GUI version available. But that would require that she learn how to use vi, which I would recommend, but which she probably doesn't want to do.