How does having a receipt constitute "proof" that she ordered the lamb? That just means that if a mostake was made it was made when the order was entered into the register, not when the order was cooked. I find it hard to beleive that a woman who, by their own admission always orders the chicken would order the lamb and then be confused as to why she wasnt given chicken, rather than say, an employee fat-fingered the order and pressed lamb instead of chicken.

She's clearly not very smart and handled the situation badly but I'd say Restaurant seems in the wrong here for calling her a liar rather than just apologising when it seems more likely to be their own fault.