Are you on cfw? If so, congratulations, you may have just gotten your friends account banned.
Gamesharing requires you to make a new user account on your ps3.
You make a new user, login to psn and download. As long as that user remains on your ps3 you will be able to use the dlc.
I'm not sure of the exact details. Look it up.
STOP! Before you ask questions... READ the first post! RTFFP = Read The F*cking First Post
Yes, A 3.55 "jb" (cfw) is available, no >3.55 "jb" or cfw yet.
Yes, A 3.55 downgrade is available. But not to lower than stock firmware! (What it came with)
No >3.55 downgrade is out there yet.