I was kind of surprised not to see this posted sooner, but my assumption was that everyone realized the huge bill he's stuck with is probably teaching him a better lesson than anyone's advice. Getting screwed a couple times taught me my lesson better than anything else.You put your credit at risk for this guy? I know you feel bad about it but why didn't he get his own phone? Because he had bad credit and now you will too if you don't pay the bill.
+1.Try to work out a payment plan, and oh yes, take your so called friend to small claims court and sue him for his part of the bill.
One thing is for sure, you're not going to improve your situation if you just skip out on your bill.