The OP hasn't asked the right questions, IMO. For most startups, the thing that causes success isn't just great hacking talent. Heck, a lot of startups really aren't solving a software problem so much as they are solving a market problem.

But even if it is a really hard coding problem that you need a star for... The big question is whether he's an entrepreneur or not.

Questions to ask him (and yourself):

1. How much traction would it take for you to quit your day job? 2. Which evenings and weekend days are you willing to set aside for this project moving forward (at a minimum). 3. How many months can you go without a paycheck (related to #1)? 4. Do you like non-coding problems? UX? Copywriting? marketing? SEO? (i.e. are you willing to evolve as a founder? I have a long post about this here: http://www.tonywright.com/2008/startup-founder-evolution/ )

In terms of parting with equity, the equation should come down to risk/value. If you're both committing to spending 10h per week on this, then he deserve a largely equal slice. If you're quitting your day job and going 40h per week (or have 300h already sunk into it), then you're risking more and deserve a bigger slice.

Be careful about overvaluing what you've done so far. A startup is a multi-year commitment. A few months of weekend and evening time is a tiny percentage of value creation compared to what's been contributed at the end.

This is good advice, but you need to be sure that the person is "co-founder" material. I have personally been involved in startups where someone who was a top-notch coder, but a horrible "businessman" was given co-founder influence on the company (they never had an official co-founder title, but effectively had the same ability to impact the company and make decisions on things).

Some people are brilliant at implementing a concept, but utterly horrible at pretty much every other aspect of running or building a company.

Just because this person would be instrumental in getting the working code going does not mean that they are at all qualified to shape the business...

What's his story?

I tried Googling it, but it's hard to sift through all the fluff.

Created wikipedia.

From his wikipedia entry: "Writer Rogers Cadenhead drew attention to logs showing that in his edits to the page, Wales had removed references to Sanger as the co-founder of Wikipedia."

I was involved with wikipedia at the time of creation (and the whole gnupedia vs nupedia debate), and Jimbo was the driving force. Larry undoubtedly played a major role as editor-in-chief but from the very beginning it was clear Larry was subordinate to Jimbo (although that doesn't necessarily mean he wasn't a co-founder of sorts).