I remember when a few people owed me money and weren't paying, so I simply just passed the debt on to a good mate of mine(who you definitely don't want to owe money to) and after a few phone calls, every one of them paid that day.
Sometimes threats of violence is the only way to get your money back. Mate or no mate, if they are taking the piss, they are then no longer a real mate are they?
If a mate lends me money, they get it back as it's brilliant of them to lend you it in the first place.