Frankly, your friend will be convicted of larceny. There are only more tests courts in the US use for insanity defense. (1) M'Naghten rule. (2) Durham rule. (3) Irresistable impulse rule. (4) MPC substantial capability rule. Majority is using M'Naghten rule which requires a disease in mind and the defendant does not know from right wrong. You said, she knew she should not buy. Sounds like she knew what's right and what's wrong. No court is using Durham rule. Irresistable impluse means on top of M'Naghten, the defendant could not control her conduct. It is not likely to work. Your only chance is that your jurisdiction uses MPC rule that the defendant lacks substantial capability to understand the criminality of her conduct or conform her conduct to the law. You need a really good lawyer and a strong testimony from a psychiartrist to convince the jury.