Pay 1 year late on cell phone bill , how much affect credit score?

Just because they-phone company- check your credit score before giving you a contract doesn't mean it helps or hurts your credit score since they don't report to the credit bureaus. If they forget your debt, which I doubt, then your fine, but if they didn't forget about the money (99.999% of the time they wont) then you're in trouble because they gonna pass it to a collection agency which is gonna list the debt in your report.

Yes, his score will now suck. Since it wasn't paid for a year, most likely, the bill was sold to a collection agency. I highly doubt AT&T held onto this account past 6mos. He would have a collection account on his record, and even though it now says paid, it will be affecting his credit for at least another 7 years. The first 2 years are the worst, but, it will remain there for some time.

You are not a very good friend.