Buy her the most expensive drum set there is and you'll be set for life. Whenever she asks you how much a guitar or piece of gear was, you can respond with "half of what the drumset I bought you cost". Or whatever fraction it ends up being.
We're late 50s and have nothing to spend money on anymore.
I read all the replies, and never saw you or anyone else say....."Does she already know how to play drums?"She mentions it briefly, but often.