How about telling her that this is one match you really do want to see, and that you are willing to treat her and her mother to a lovely lunch in town? Or else, explain about the importance of the match, and arrange MIL's dinner for another day?
What time is the "late" Mothers' Day meal? Can you squeeze in both, as a compromise? You'd be back from the Amex around, what, 4.30-5pm? If no compromise, such as this, is possible, then I'm afraid mother-in-law wins. Can you afford to insult both the mother-in-law AND the wife?