I've got a very simple solution: just give the gecko away. Period. Once you do all hell will may break loose "your a bad brother! How dare you save that animal from improper care!" Or something of that sorts. I have the same experience at my work from time to time (doing others chores) and it's not fair for you to have to do extra. I get that like what was mentioned above, it is easy to just feed and water it since it's a leopard gecko after all, but you didn't sign up to take care of someone else's animal.
Just give it away or take it back to the store. Lie and say it died and you buried it so she wouldn't have to see her pet dead. This way it doesn't happen in the future (in my experience it will happen again) and she doesn't get another pet without planning to care for it.
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