If you have ever borrowed a large sum of money from a friend you probably already know the kind of trouble you can get into. If you haven't, let me break it down for you so that you do. It generally starts off with you venting to your friend about your situation. Then they, being your good friend, try to offer help. You, needing the help, take it. Maybe you don't take it at first, maybe your sense of pride causes you to think on it for a bit, but eventually you do.
At first everything is fine. But, lets say you are really in a financial bind. 5 months have gone by, and despite your best intentions you have not been able to repay a single penny. So now what starts to happen? That owed money will sit in the back of each of your minds. You and your friend have a fight, guess what will come up in that fight. In general conversation with your friend they mention the fact that they are still owed money. As time wears on you start to feel guilty that you have not paid your friend back. Just being around them makes you feel guilty. You start talking to them less to avoid confronting them on it. Or you avoid them altogether.
In an even worse scenario, lets say your friend starts to run into financial trouble. Then what? Guess who they will go to, and if not paid back right away, start to blame.
All of this; guilt, avoidance, non-communication, results in only one thing. No friendship. The same reason money can kill a marriage is the same reason it can kill a friendship.
Here is the secret to keeping a friendship in this situation from going down the toilet. Keep talking about it. When you owe a friend money, don't let the fact that you owe be the unspoken elephant in the room with every interaction. Stick to a repayment plan, and if you can't follow it, communicate that fact. Make a new plan if you must or form a new agreement or do whatever you need to do to handle the situation, but handle it. And communicate it.
Borrowing from a friend when you are really in a time of need can be a lifesaver. It can be just the safety net that helps you out of a bind when you need it most. But personally, I consider borrowing from friends a last resort. But if you must, work out a repayment plan beforehand. Make sure this plan takes into account what to do if payments can't be made. Any expectations on when things should be paid off and if your friend is expecting a little extra for their own troubles (interest.) Being in debt is itself a treacherous mind game. Adding onto it the multi-layered emotions of a good friendship can turn it into a nuclear bomb. So remember that no matter what happens, have a plan, stick to it and always, always communicate what is going on.