I'd recommend starting out on GameFAQs in all honesty. It's where most of the indie guide-writers start their journeys.
It requires dedication, perseverance and a methodical approach. You need to decide what it is you want to help the gamer over-come and cater your guide to those needs.
Therefore great care in how you lay the guide out must be taken, write what needs to be written and nothing more (clarity is essential in game guides).
As well as that, you need to be actually playing the game you're writing for over and over. Testing out mechanics, discovering the best way to navigate levels... where all the bonus items are... as much as you can.
It'll take time to perfect, but if your friend keeps at it, it should become a very enjoyable avenue of gaming.