While I feel for the MotB, it’s not the MotG’s responsibility to make sure that everyone who has an allergy – severe or otherwise – is accommodated for. Cross contamination is always a possibility, but if the MotB is old enough to be the mother of someone getting married, clearly she’s figured out a few tricks for staying alive in this world. The MotB can bring her own food, bring an EpiPen (if that would help), refrain from eating at all, or simply speak to the waitstaff and explain how important it is to avoid cross contamination. However, the rehearsal is about the bride and groom, not the MotB. How many children the MotB has is really irrelevant.