Mmmm a tough one.
I remember spending Â£200 once on some disc brakes. there was a knock on the door and my neighbour wanting some information about starting to mtb. I asked her what her budget was, and found myself politely explaining that for that sort of money the bike probably wouldn't last long on an old disused railway track, let alone go up a hill ( Â£100 ). I showed her the disc brakes and she decided to give it a miss.
Anyway, we all have to start somewhere, and I would get a second hand frame and if you could help out on parts to put a decent bike together.
We must do everything to encourage more women into mtbing.
That extra Â£100 would make all the differnce to her having a more comfortable ride ( a lighter bike), with componants that lasted longer.
I myself have an On One frame ( 14" ) and call it the mountain goat.
If you live near Stockport, she is welcome to try it out.