Is it illegal?
Me and my friend over xbox both have our accounts downloaded on each of our xboxes, he lives in Texas and I live in Canada, and I was going to use the credit card on his account to buy battlefield 3 without him knowing, i've known him for 2 years, If he finds out could I get into alot of trouble?
Well if he finds out he can report you for using his credit card without his permission, so yes you can get in trouble.