While an attorney will need to review the facts in detail to determine if the bank was aware of the extent of the authority that you permitted this person to use your card, the bank can hold you liable on the account, if it was not aware that this person was placing unauthorized charges on a card that you permitted this person to use.
If you believe that you are a fraud victim, perhaps you should consider filing a police report against this person and see if this may help you negotiate a settlement reduction. If there are any charges that you dispute, you should do it within 60 days of the credit card statement to the address for billing purposes by certified mail. See letter #2.1, in the second link, below, which are free sample letters for consumers. Even if you missed the 60 day dispute period, send a written dispute of the charges anyway.
You are most likely going to be responsible for the debt. But it sounds like you should be able to go after your co-worker as well. That wont make a difference to the bank, but you might be able to get some of the money back from the co-worker.
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