Why not just say, 'if you like it, ask your mum for it'.
Puts it back on the mum to cough up and get.
Hell I'd say to the kid, 'if you want expensive things, ask your mum to give you some money to buy them'.
The mum is doing the child no favours and I feel sorry for her, her friends and later on in life her partners, will find whining to get what you want annoying.
If the mum tries to make a point with you just smile and say 'that's great, I thought you'd get it for her if she really wanted it, I'll make sure to point anything she likes out to you next time so you can get it.'
If she's just wanting to get her child things then you are doing her a favour, if she's trying to make a point then she's buggered because you've stepped over and ignored it.
Going through cupboards and things, I'd lock them.
If she begs just say to her 'No, if you want it though you can call you mum and ask you to bring them round.'
Again puts it back on the mum to be the one having to enable rather then putting it on others.