I agree with Seusomon that mothers should be supported in meeting their infants' needs. Even though it is technically a health code violation to bring outside food into a restaurant, any reasonable restaurant owner/staff would overlook a jar of baby food and even be happy to warm it up.
The problem is that she brought chicken baby food to a vegetarian restaurant. Now, I could see an omni not anticipating the feelings of vegetarians on this issue. It's hard to always be sensitive to other viewpoints, especially when they are not your own. But this mother says she was a vegetarian for 20 years. This is where she begins to look like a trouble-maker to me. She had to know that it was potentially offensive for her to do this.
However, the reaction of the restaurant owner is also troublesome. Sometimes when I'm contemplating such situations, I ask myself, "What would Miss Manners do?" Here is my solution: as a restaurant owner or staff person, make your feelings known in a polite way, and ask the customer to accept some pureed veggies from the restaurant kitchen instead. Baby food is ridiculously easy to make. This would have gotten the point across, kept the chicken baby food out of the microwave, probably would have made the customer happy (especially if they didn't charge her), and finally, it would have made the restaurant look really accomodating to mothers with infants, which is great publicity.