I know a lot of people have said their piece, and it's mostly the same thing, and this is by no means meant to insult the op in any way, or make her feel bad. I'm sure this happens quite a bit, but I thought I'd add my two cents in too...
I read the whole thread, and it's quite disturbing to me, to tell you the truth. I too am trying to start up, and get business, and I am very proud of what I do. I have made cakes for brides that don't know me, and they've trusted me for no reason, but that's how you gain business and respect in this business. I think it's ludacris to blame the bride for trusting someone who has advertised that they make wedding cakes, and agreed to the fact that they can make what the bride wants. Even if she said she was "practicing" if she was confident enough to say she could make the cake, the bride took that confidence as ability. If I were in the bride's shoes, I would be very mad, and not that I approve of being rude to others, but, in some cases, there's nothing else you can do but be rude. This was her wedding, and she has the right to want things her way, especially if she paid for them to be that way. I really think that it would have been better to not have a cake, or it not be vegan then to present a cake that is destroyed. The couple has every right to demand a refund, they didn't get anything that they asked for. As for eating the cake...why was it brought if it wasn't meant to be eaten? I think the complaint would have been the same if she had seen the cake and thrown it in the trash...then you would have said I went through all the effort to get the cake there and she just threw it away.
Ok, too much ranting, I guess. My thing is this, I think like someone said before, at some level we are all still practicing and learning, but part of that learning is knowing our limits. Wedding cakes are not meant to be experiments. I totally agree that a full refund is due, even if it's in installments. I know you don't live in that town anymore, and aren't expecting orders...but word in this kind of business travels fast. And you never know who this person knows. And as a college student, court fees are the last thing you want to fit into your budget. So, chalk it up to a big lesson, and pay her back. Don't use her rudeness as an excuse...the cake was still your responsibility and you didn't come through, so you have to remedy the situation.
You both had a bad day, but if you let this go on, she could make a lot more of your days miserable.