I had the same response that Mollie had when I heard this story for the first time: my jaw hit the floor. Man, what an awful beginning for the mother-in-law/ daughter-in-law relationship . . . it's hard to imagine ever recovering from a public slap in the face like this one.
I take that back - if, upon being asked about it by Mimi, her Future Daughter-In-Law (FDIL) had said something along the lines of, "I'm so sorry. It was a stupid oversight, none of us caught it in time and the invitations have already gone out. I feel just awful," they might have eventually been able to laugh about it together (in 20 years or so).
Since FDIL basically said, "Yeah, whatever. It was your stupid son's fault anyway. Get over it already," it seems unlikely that FDIL and Mimi are going to get to start out as friends.
Unfortunately, in a situation like this I don't think there's much of anything you can do that won't make things worse. The fact is, any public action Mimi takes will only draw this "oversight" to all the guests' attention; probably no one but Mimi's own friends have really realized that her name is missing from the invitation. Everyone else is extremely unlikely to stand up during the processional and shout, "Hey! Why is there a Mother of the Groom walking down the aisle? There was nothing about this in the invitation!"
For Mimi, this is one of those rare opportunities Life sometimes throws you - the chance to look like The Better Person by not doing anything at all. If Mimi dresses beautifully, is gracious and welcoming at the wedding and reception, and is seen often smiling lovingly on the happy couple she will be seen by one and all to be a civilized, well-behaved paragon. If FDIL is a screechy, nasty Bridezilla, anyone contrasting the two women will clearly see which one's been house-trained.
Sometimes there is a bit of jealousy, between a mother and a daughter-in-law. Some mothers expect to keep their places as #1 in their sons' hearts, and some wives feel that they must compete with "the way his mother did things." Though Mimi would certainly be justified in raising a stink about being dissed in such a thoroughly public fashion, it would place her son in a tug-of-war squarely between the two women. In this case I think Mimi should turn the other cheek, zip her lip and smile, smile, smile. While she's at it, she can pray fervently for a steaming barrel of fresh-laid karma to catch up with FDIL on the honeymoon.
Which is not to say that I, like Mollie, don't love to consider alternatives.
1. Her wedding gift to the couple could be a framed, enlarged photograph of Mimi holding Son as a very small boy - for each room of the house.
2. Her wedding gift to the couple could be an official, calligraphic copy of Son's family tree - with Mimi's name circled in yellow highlighter.
3. Mimi could wrap her face for the wedding a'la The Invisible Man.
4. Mimi could pass out Mimi masks, to be worn by every female sitting on the Groom's Side of the church.
5. Mimi and her husband could book the adjacent cabin on the honeymoon cruise ship. Surprise!
6. When grandchildren begin to arrive, Mimi could give them whistles, "laser" pistols, drum sets and other noisy gifts on every conceivable occasion.
7. Mimi could sneak into the kids' apartment when they're not home and remove every other stitch of the seams holding the skirt of the wedding dress to the bodice off the wedding dress. Then she could pay the ring bearer 20 bucks to jump on the bride's train as she's leaving the altar.
8. Mimi could slip an ipecac mickey into the bride's special champagne-toasting flute.
9. Mimi could stand next to the bride in the reception line and get FDIL's name wrong whenever she introduces her.
10. Mimi could wear the same dress as the bride.
PS - I'd like to publicly thank Bender for marrying a sweet, thoughtful, smart and funny girl so I didn't have to worry about anything like this!