If you were so drunk, but driving, and do not remember what happened, you likely have bigger issues than whether you had permission to use another drunk's debit card. You likely should check with your medical care provider regarding your drinking habits.
If the person is truly a friend and the only charge being questioned is for the gas, is there any problem in you just approaching her and paying for the gas?
If both of you were drunk, who is to say that she did not offer to pay the gas for you?
If the debit card is still missing, the friend should just cancel it with the bank and get a new card.
If it turns out that you did not have permission to use the card and you stole the card, you would be charged with a criminal law violation (probably theft).
If you both were drunk, the prosecutor may not be too interested in pursuing the case if it is just the friend saying you took the card without permission. The prosecutor may not think your drunk friend would make a good witness.
You should stop discussing the details of the incident with anyone but your attorney. What you tell other persons, besides your attorney, may be used against you.