I have a friend whose first language is Mandarin and I will be helping her improve her writing skills in English. Does anyone have any suggestions on what materials or method that I should use?

Practice reading and writing with the dictionary in planet of apes.
Practices make it perfect in planet of apes.
Ever wonder how ET got his finger tip burning with light in doing his writing in planet of apes?
"I must not talk in class" with 500 lines in planet of apes.
"I must finish my home-work" 300 lines too in planet of apes.

Well, I actually quite agree with the answer given by djdrunkbabii.

I speak from experience, you know? My mother tongue is Mandarin and I once had quite a hard time dealing with English. I've managed to overcome that problem though.

What I did was watch more English cartoon or dub-ed anime and read lots of English books, mostly novels, since I'm more interested in them than newspapers. I started from the easier ones and slowly began reading harder ones. The dictionary helped a lot.

Tell your friend to do the same, maybe? That's how I helped some of my friends, by telling them my own experience...

Good luck to your friend!

First of all, try to make him watch as much TV as possible, especially the news and cartoons (Disney Channels, Nickelodeon, PBS Kid, Cartoon Network, etc), This is because news reporters speak English very fluence and clear; same with cartoons, otherwise children wouldn't be able to understand what they're saying. Secondly, just give him books, and let him read them. Books don't have to be a love novel or anything, they can be about anything, a cook book or a piece of newspaper would help him through time. Thirdly, time and practices are what he needs. His English will be improved as long as he socialize with people in English. He needs a lot of practices if he wants to speak English fluently within one year!! Plus, not everyone can do this, it depends on the effort he is willing to put in and his "age" too because sometimes it takes old people more time to learn a foreign language!

The same way anybody gets to a concert hall, PRACTICE, PRACTICE, PRACTICE.
But first, help her by introducing her to some authors that she might like, because if she like's the author, she will WANT to learn more and more.