I agree with Richard's point of defining clear expectations, and Brent's point of treating the potential partner as a hire.
I started a company shortly after college which has experienced a decent level of success. My first two hires were college friends whom I hired for their character strengths, rather than a specific skill set or role contribution. I eventually gave them equity stakes and treated them as partners. This has worked well with one, and not well with the other.
I think the individual you are considering needs to have an entrepreneurial or leadership track record, and bring a clear skill set or role to the table which he/she can perform better than you. For example, if you are business development oriented, he/she is the financial mind. If you are just bringing on a friend for general help, it is going to strain the relationship and you may still be on your own for the primary entrepreneurial roles in your business.