Having read the all the suggestions, this is the one I'd recommend. Partnerships are more bother than they are worth. There are a lot of IF's: If he brings in the business. If he does a "good" design. If he manages the client and responds to his calls. If he pays you on time. If he brings in clients that pay at the top end of the scale. Then he might be worth 50%.

My impression is that your friend is trying to profit from your hard work. I don't see the amount of time as being 50/50. So you need to factor in whether his contribution is worth the 50%. In my experience you'd pay 5% for an introduction. 15% - 25% for the invoicing, collecting, account management. So really depends whether the design work is worth 35% or more.

Another arrangement is that you get back to him and say, that if he makes the introduction, you take on the client and pay him a commission of 15% and bill the design work to the client and keep 15% for admin costs.

A partnership or company is the last thing that I would suggest. Too many things can go wrong over time. Believe me, it took me 4 failed companies before I realised that it's impossible to reconcile expectations, especially when people have jobs, social life and other ever-changing issues in their lives.

Good Luck!