It's nice to be friends with customers. From a marketing perspective I encourage you to have as strong a relationship as you can but when attention turns to debt collection, it is a different matter.
Some small businesses shy away from using formal debt collection procedures with friends and are even reluctant to ask to be paid.
It's as if asking for the money you are owed will damage your friendship.
"They'd pay you if they had the cash wouldn't they?"
Well perhaps not.