Totally agree, never mix business with friends.
I started into this industry with my best friend, when everything started to happen, and quite rapidly, it was difficult to maintain control, when your friend isn't following or turns out to have a different vision to your own. After having quit my previous career and having agreed the new lease, we mutually agreed for me to go it alone, so as to keep our friendship. Ofcourse this meant that I had to proceed with half the funding, but at least I have full control to go at my own pace, with the moral and voluntary support of my friend.
With any partnership, you would have to draw up contracts so as to have a clear, definable understanding of what your obligations are for the business and what you will get for it and when.
This is always messy, as you want to get the most out a partnership, so having such things out with friends is obviously stressful thing.
Ofcourse you may be one of the lucky ones who is at one with your friend and are able to agree each other's roles and trust each other.
What ever you decide, I wish all the best in your new venture.x