is she someone you see quite often or a neighbour?
i would ask for it back because you need it for tomorrow as you're taking pictures for a friend at a kids party...or something along that line. hopefully she will give it back so you don't let the kids down. i know its a lie but its worth a try
hope you get it back.
"sorry went home and found it is broken"
"sorry my mum, brother, sister etc has it"
"I've lost the dam thing sorry, my home really untidy at the moment lol"
I would tell them that the camera belongs to someone else and therefore you cannot loan it to them. Say you misunderstood the question when it was first asked.
I would lend my camera to some I could trust to look after it , but not to someone I hardly know. So what happens if it gets broken or lost ? tell them to get a disposable camera which comes with the film they only cost �1.99.