Ask HN: How a business co-founder can help me?

Hi guys, I'm building a product and there's a close friend of mine who is currently occupied with business and marketing. He is a great professional and I was wondering what I could possibly win from asking him to join me? What should his role be in the whole situation? Since I will be handling most(all) of the technical issues how can I give him 50% of the weight from the whole situation given that I don't have any investors/working prototype yet?