Interesting thread, there seems to be a lot going on in this situation. I just wanted to suggest that if Mom deliberately tries to ruin LK's belief in Santa--such as by saying, "There is no Santa, your mom bought you that," which I think is very mean-spirited by the way--it won't necessarily work. I would guess that LK would be perhaps confused, maybe upset if her grandma speaks to her in a scolding tone, but I don't think her world of make-believe would shatter into a million pieces right then. If she even remembered it for very long, she might ask the OP about it later, and the OP can assure her that different people believe different things, and in their house they have Santa, and other appropriate things.
Note that I am not diminishing the problem; I also see it as major boundary-crossing by the mom, no matter if her intentions are good in her mind. And even if "all" that happens is that the child is upset to be scolded by her grandma, I think that's still bad, because it's unnecessary and will make the visit unpleasant. But, my personal take is that it's unlikely to really be something that the child will remember for long--I think it's much more an issue for the adults.
FTR, I come from a family that was Santa AND religious; my dad is a minister. I had the Easter bunny, tooth fairy, etc.. We weren't "militant Santa," though (not saying the OP is, just that I've known some families like that). To us Santa was just "playing pretend," no more a lie than running around pretending to be superheroes with towels for capes, or letting a little kid think a card trick is really "magic."