I think it depends on her skintype, her colouring and her personal taste. Does she like shimmer or more discrete makeup?
I don't think you can stick to one brand anyway - my favourite mascara is from Maybelline, and that is the only thing I own from that brand. Honestly, I think it's all crap except for that one mascara
MAC is great, but their foundations make a lot of people break out. Their mascaras aren't supposed to be very good either.
I don't think that there is such a thing as a perfect make up collection. She needs to find the products that suits her.