I've done a lot of business with friends, especially considering they needed my assistance. I've done pro bono work, work for non-profits at discounted rates and other options. I usually get a deposit on work to be performed, but recently I've learned that some "friends" you just can't trust.
A logo needed for a new business venture was requested by a "friend". I obliged, offering a reduced rate. However, I didn't ask for anything down. That was my critical mistake.
After a few hours of work, several revisions and email correspondences, I was assured a check was being dropped off after completing a couple of final versions last week. The alleged check (or payment) never came. Come to find out, the business venture never got going and this individual thought that since the logo wasn't used or didn't get final approval, he wasn't obligated to pay for my services. However, he could afford to attend a rock concert and expensive dinners (via his Facebook news feed) while shafting me on my payment for services rendered.
So keep this in mind doing business, friends or not, payment is required at the time of service.