Hi Duet
I worked for a credit card company here in California for one year, and I know only the authorized user can make changes on the card!
They may all be different this was Wells Fargo Bank; therefore, may be best if you just call up and ask that as a "generic" question, as they sure won't give you any information on the account even though you know everything there is to know about it!
I, frankly, am surprised that the judge ordered you to pay your half because normally the person whose name is primary on the card is the one responsible for the payments. If he authorized you to use it, then he is still responsible for the payment. Things change all the time in credit card companies, and it's been a few years since I worked for them, so God knows what th deal is now. Just call them!
Besides, what if you pay your share and he decides not to pay his??? Your credit will take the hit anyway! There's another question to ask.
Hugs, Devastated