My heart broke when I read your note. I would hope that a parent would be excited and proud and encouraging when their child learns something important like the alphabet. Giving her the benefit of the doubt, perhaps she was feeling guilty that she hadn't taken the time to do it herself, or perhaps she's just too tired to cope with an excited child after a long day at work.
The only thing I can think to do is maybe to tell the mom that you wonder whether her daughter is driving her a little crazy with it, and offer to give the child a set of plastic letters so she can play with them by herself, without bugging mom.
Maybe reopening the topic in a sympathetic way will create an opportunity for the mom to say, "go ahead and keep doing this," or "I wish you'd just stop." If she doesn't raise it, maybe you could just ask her directly.
I would feel exactly like you do. Whatever the mom says, your heart is in the right place. Good luck and let us know what happens.