As author of
Wild Spellcraft, I will gladly answer any questions you might have. First off, if you don't want to substantially change the way magic works, I would advise not just jumping into Mongoose's
Chaos Magic. It's a fair system if you want to be really experimental, but the results on your character can be a little hard to deal with. Read the reviews of each before you make your decision.
Wild Spellcraft: Available as a $5 pdf file at RPGNow.com, or in about a month for $13 as a book in your local gaming store. See reviews at
http://www.enworld.org/d20reviews.asp?sub=yes&where=currentprod&which=N201
Chaos Magic: Available for $15 at your local gaming store. See reviews at
http://www.enworld.org/d20reviews.asp?sub=yes&where=currentprod&which=ENCM
As for the myriad of fan-made versions online, I believe that though many of them have some nice ideas and inspiration, none of them are fully detailed to cover the wide array of events that might occur in-game. If you just want a very brief set of rules to play by, I can upload some of the more basic Open Gaming Content portions of
Wild Spellcraft, but overall I think you're better off getting a full book.
Can I ask a little about who this player's character is going to be, and why he wants to play a wild mage? If he wants to explore the dangerous side of magic and risk his life, you might want to look at
Chaos Magic, but I'd say
Wild Spellcraft is better if you just want to inject a little variety and spice into your character's spellcasting.
Chaos Magic is best when used for it's main purpose: to be a flexible spell-design system (albeit with a lot of risk). Wild Spellcraft is best suited to represent wild and unpredictable magic. Get whichever one you think fits the style you'd prefer. Of course, as the author, I'm definitely biased, but I'd like it if you could get both, and compare.
Mr. Sprange or Mr. Witt, feel free to chime in.