I would suggest to your mom that she consider the places that they go and decide on one or two that are the most likely to be successful. Some factors to consider are the overall atmosphere of the place and friendliness of the staff. Choose a place and time that aren’t too busy and crowded. Plan ahead for contingencies: a long wait can lead to restlessness, and too much busyness might become overstimulating. Find a place to sit that’s fairly quiet and where an easy exit can be made should dinner need to be cut short. Take a snack or small activity for the wait time. Perhaps a quiet word with your server would be helpful so she understands the situation. If your mom goes to a regular place on a consistent night, hopefully she can establish a relationship with the staff and not have to explain all the time. Your mom might also feel more comfortable having a third person present.
Additionally, some caregivers travel with wallet-sized cards that they can hand to staff asking for patience. See the article “Taking Your Loved One on An Outing” to download your own set of Companion Cards.