Yes, this messes up your debt/income ratio. This account will show up as a JOINT account on your CR. It will be as if you opened this account together with your friend. you will be LEGALLY responsible for this account just as much as you are for all your individual accounts.
based on personal experience, i would advise you against co-signing for anyone. i co-signed for a good friend (or so i thought at the time) in college for a car lease. his score was down in the low 500s and mine was over 760. and of course not only did he have about 15 late payments over the life of the car lease (30 days, 60 days, 90 days), he also didn't have money to pay the fees at the end of hte lease and i barely got him to turn the car in on time....that only happened because i contacted his mom and she yelled at him. anyway, due to his late payments, my scores dropped into the 500s. i am working my way back up (as the lates get older), but it basically just takes time. i have NEVER paid any bill late in my entire life...and i have plenty of bills to keep track of. and i got screwed in my credit scores just for trying to help out a friend.
