Please, please, DO NOT DO IT!!!!!!!! It can take years to pay off loans, who knows if you will still be friends at that point - I married someone who was still paying off a LOT of money that was loand for an ex. Now as the new Wife its my debt. Not good. Not happy. And we are now living broke while the person who borrowed the money is living rich an not paying a penny. Please do not do it!!!!!
Now, if after all that (and everyone else's similar responses) you insist on doing it, PLEASE get a written agreement between the two of you, stating they owe you the money, and all insist and fees involved. If you can do get a lawyer to look at it. Get it notarized. And then, prey that they won't f'ck you over. It still go very wrong. Keep a good written record of every payment made. Tape record any telephone calls regarding the loan where your friend agrees they owe you this money. This is so risky and you really are risking your future and your credit doing this.
Instead, I would suggest you tell your friend to read the post below I made on another question. Good luck, and PLEASE DONT DO IT :)
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For your friend to read:
Best three that will approve those with bad debt, and are direct companies (so you don't have hundreds of companies contacting you just to say 'no') are:
Spring Leaf
Republican Finance
Argon Loans
Do note that all three charge high interest, obviously as you have no/bad credit and no-one else will touch you, but I've found they are all quite fair, as in, if you pay them, they leave you alone and they are the best I could find if you have very poor credit.
However - before even going here, I suggest selling what you can (check out eBay), getting a part-time job, calling up your electric/gas/TV etc and seeing if you can get a better deal to save money, and, very importantly, doing a proper look at your money for the next 6 months to work out where you can save money. If things are really bad, look into bankruptcy, seriously, take a look at it before just dismissing it.
Also - Please - STAY AWAY FROM PAYDAY LOANS!!!
Good Luck, and remember to keep it real, if you can't afford it, don't do it.