Hello and thank you for entrusting me to assist you. I am terribly sorry to hear about your mother's fall and injury.
To have a viable claim for premises liability, a claimant must typically demonstrate that their injury occurred as the result of a condition on the property that created an "unreasonable risk of harm," that the business owner knew or in the exercise of ordinary care should have known of the dangerous condition, and that the business owner should have realized that it presented an unreasonable risk of harm.
Put another way, there must be something inherently dangerous about the property of which the business owner was aware or should have been aware, for liability to attach. Obviously, the business was aware of the step, so, the question is whether the step, without a sign, bright paint, or other form of warning was "unreasonably dangerous." This will turn on some very specific details (lighting, amount of foot traffic, size of the step, etc) and ultimately is a "question of fact," meaning up to a jury to decide in consideration of the unique circumstances. There is no black and white definitive answer.
You can bet that in addition to arguing that the step does not constitute an unreasonably dangerous condition that the restaurant will argue for comparative liability--that is, that your mother failed to exercise due care by not looking down and accordingly that she bears some "fault" with regard to her injury. The restaurant will likely also argue that while the step may have been unreasonably dangerous for your mother, it is not for an individual with vision in both eyes, and as such, the danger to your mother was unforeseeable.
This is not to say that a claim for premises liability would be untenable on the facts you describe. A claim such as this could very well be worth something, but it would be an uphill fight and would likely require litigation, especially if she is seeking significant financial compensation for her injuries.
Should you be unable to settle with the insurance company or they outright deny liability (a likely possibility), you would be wise to hire an attorney on your mother's behalf to pursue the claim. See here to locate one near you: http://www.avvo.com
Attorneys frequently take slip and fall cases on a contingency fee basis, If you don't know, a contingency fee arrangement is one in which the attorney receives a portion of the client's settlement or award as his payment, typically 1/3 of the total amount. If there is no recovery, the attorney does not get paid. The client never pays until the settlement or award is obtained (except perhaps to cover the filing costs for his claim).
Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes to you and thank you so much for coming to Just Answer.