I don't know a single thing about wedding cake protocol but one thing that I think you should be very careful with is any suggestion that just because they ate the cake, they aren't due any refund. I AM NOT SAYING THEY *ARE* DUE A DARNED THING! I am just saying, I respectfully disagree with the idea that you should use that as the basis of how you handle the issue. I'm trying to look at it from the customer's standpoint, and let's say for the sake of argument the thing was about 4 secs from falling over so before they even started the reception they cut the cake FAST and then served it after dinner. Well, that's a huge disappointment for a bride, since their cake shouldn't have been falling over. But if it was your wedding cake, wouldn't you eat it anyway? I mean what else do you do, order in twinkies?
That said, I strongly believe the burden of proof is on them to demonstrate that there was a problem with the cake. The cake looks GORGEOUS and it's true, what happens to it after you leave can't be entirely your fault. Unless you think it is your fault, if you think the globe supports didn't do their job and the cake was about to become the leaning tower of Pisa. There have to be 10000000 photos of that cake, before during and after the cutting ceremony. They need to pony up.
As for suing you - it sounds like she's just trying to be a *****, thinking if she threatens you she'll "win". I truly hate these types of people. They have no respect for others, and no concept of reality. She should absolutely positively be showing you photos of this "fallen apart" cake if she thinks she should get a penny more than what you feel is a fair "customer satisfaction" refund (should you choose to offer one). But I think what you yourself said kinda answers some of your own question: you already wanted to give them a refund because you felt the cake was a little leany. Don't reneg on that just because Mom's a jerk, but do not under any circumstances imply that you think anyone is "entitled" to a penny; any concession you offer should be a goodwill gesture. And a lawyer-OMG yeah right, she ALREADY talked to an attorney about a CAKE?? What a joke! I can totally see WHY you would prefer to offer nothing, just out of spite, because clearly the lady is trying to rip you off.
Totally agree with LukeRubyJoy that you should never imply that any cake is any less than perfect unless it is blatantly a disaster, so now you could just say "Listen, the cake was fine, I made sure of it when I left, but in the interest of customer satisfaction I will offer you an $x refund" or some future deal like cakes-r-us suggested. The lady sounds like a real *ahem* but that shouldn't change your initial instinct about the right thing to do. Forget about what a jerk she is: just offer what you know is right. But still insist on photos first, that is insanity to take her on her word, esp. since she has proven to be a total liar.
This is all my extremely humble opinion, again with 0 experience as a baker...I'm trying to look at it from the customer's point of view (i.e. the BRIDE) and think: what is my job as the customer in resolving this problem, and what is my cake designer's job? Oh while i was typing this some more posts came in, including from you, including that the bride said it was fine. I got news for you, she just completely wiped out her psycho-mom's ability to "file a lawsuit". What a surprise that would be when the bride was called in to court as a witness against her own mother LOL!!