@raynbow- Your mom will definitely need to get some orthotics that have good arch support and heel cushioning to put inside the high heels shoes she buys to help prevent her fasciitis from flaring up too much. However, there are certain types of formal shoe features that should be avoided and those that are better than others for people with this problem.
First of all, your mother should avoid heels that are higher than 2 inches. If they are too high, they will definitely throw her arch off to the point of causing heel pain.
Next, she should look for high heels that don't have extremely pointing toes. These tight-fitting shoes are almost impossible to fit a good orthotic insert into, and also decrease arch support.
Finally, as she shops for the best shoes, tell her to look for rounded toes, square heels, and soft insoles. The combination will give the most comfort for her condition, yet the style she wants for her special event.