She has to sign a contract.
“I, *insert name here*, fully agree that this gold nail polish is not my property. I own up to anything that happens to the bottle or handle while this polish is in my care. Should anything bad happen to this bottle in my time of care, I swear to either pay Bianca back full price or buy a replacement bottle of the same or a similar color.
And tomorrow, when she signs said contract, I will have an eyewitness to see that we did in fact negotiate this contract in full.
I take these things seriously.