Is it permissible or forbidden to benefit from the free points offered by a restaurant to customers, which they have not used, knowing that this benefit does not harm the customer, and what should be done if it is forbidden?

There is no harm in the restaurant giving points that convert into a discount for buyers through the application. However, if you are an employee at the restaurant and used the application on behalf of the buyer, it is not permissible for you to take this discount because it belongs to the buyer. Your taking it would be an unjust appropriation, and it is not permissible for an agent to sell to himself unless authorized by his principal. If you were an agent for the buyer, it would not be permissible for you to take the discount without his knowledge. Your duty is to repent and return what you took unjustly to the restaurant by any means, without obligating you to inform the restaurant owner.