Slip and fall in McDonalds with "wet floor" sign, resulting in broken knee, lost wages. Insurance Company denies any negligence

North Carolina is a tough state when it comes to premises liability cases. If it is determined that the victim is even partially at fault, there may not be a recovery. You should still contact a local personal injury attorney to discuss the facts including the visibility of the sign and the condition of the floor.

The answer is not cut and dried. An attorney will consider many factors when deciding whether or not a case has merit. You did not present nearly enough facts in your post to allow anyone to give useful advice. Your wife should contact a local personal injury attorney, free consultation, and discuss this case with that attorney. The one thing I can tell you, and I don't think there will be any disagreement on this, is that an insurance company telling you that you do not have a case means absolutely nothing.

The location of that sign will be important and other factors to determine if there is a case. Cameras are typically available as well. Contact an injury attorney to discuss further

The sign being out is also evidence of their knowledge that the floor is wet and thus, possibly dangerous. The problem is that North Carolina is a "contributory negligence" state which means that if you are even 1% at fault you may be barred from recovery. The sooner you contact a personal injury attorney, the better. The attorney will need to investigate the circumstances of the accident and determine whether you have a case.
Don't go by what the insurance company told you. They don't want to pay, they want to get rid of you. But, when you have a lawyer on your case, then you are not so easy to get rid of!