The sad truth is "GET IT ON PAPER" - too many friendships have ended when it came to choosing between money and friendship.
Judge Judy has taught us that this is VERY true haha.
Also, if you're both new to starting a business definitely do your homework. Most cities have a chapter of SCORE in them which offers free business counseling. They can help you with analyzing things and writing up a business plan, researching investors, etc.
You also need to realize that all businesses take time to blossom if successful...you're putting extra money and effort into it for quite a while before you're turning a profit. Break-even profit projections are often most realistic, so if you and/or your friend think that this will be a quick success and easy way to be your own boss, think twice.
Another big thing is doing intensive homework on competitors in your area when you plan to set up a storefront (or even if selling things from home). Who else does what you're planning to do? Businesses, medical providers, etc.
I used to be a Vocational Rehabilitation Counselor and had to learn a lot more about all of this than I thought I'd ever know as a Social Worker, haha.