I believe that truly good friends can enter into a business together. I have seen it first-hand several times, and in every single case, the friendship changed.
In all cases, they have remained close, but the relationship changed.
Business partners will often not see eye to eye on certain issues; this will cause differences in opinion and usually put a strain on a relationship.
Should you do go into a business with a friend that you absolutely trust. You must make sure that everyone knows his position and job. You must set clear expectations and who brings what skill, knowledge to the table. It would say that everyone knowing their job and proper communication is essential.