if she really wants mac, then why don't you buy her one mac brush, rather than a whole set of something she doesn't really want, you can tell her that you couldn't stretch to the whole set, but you'll help her build it up over the years. That's what I'd want anyway, I'd be sad if someone wasted their money on the cheaper option as it wouldn't stop me wanting the better one.
I was in Space NK the other day and the SA told me that there was going to be a very good offer/discount on Stila (now discontinued brand for Space NK) makeup brushes and she intended to stock up. I 'think' the sale starts end of next week.
Just looked on their site - I'd get 1 brush and a cheaper item such as the sharpner duo (£3.45).
I love Mac - the same thing happened to me last year. My sister bought me some make up somewhere else because she got more for her money and it was a waste because I havn't used it because I really like Mac make up...