She wanted to leave at a certain time, she did so. And that's literally the entire story.
Exactly. and the OP asked was it rude to do that.
I would say the answer is a unanimous yes.
She sounds like an entitled, selfish, mannerless teenager. Just because SHE wanted to do something, does not mean it is acceptable behaviour. But then it seems if that's what SHE wanted, that's what SHE gets. Fuck anyone else.
And if she couldn't figure the possibilty of an hour (wasn't even multiple hours!) delay at a wedding ffs she is either off the chart selfish or incredibly thick. I think either of those puts pay to her being a brain surgeon performing life saving surgery at 6.30am and more in the camp of rude brat who needed to make sure her hair and makeup were perfect for her shift at the local shop/cafe/office. Or the fact she had plans at 8pm to go out on the piss with her friends/BF and those trumped any respect or decency towards the bride and groom.
And the MIL simply wanted her to stay, yes she got up from the table and followed her outside to talk her granddaughter into staying (because she probably couldn't fathom how someone could be so rude as to leave mid meal). Her father also got up and followed her out because she apparantly needed walking to her car.
I can catagorically say that if I'd coughed up £100 for a meal at my wedding and a teenager left mid meal for absolutely no reason whatsoever, except needing to 'go to bed at 8pm, so they had time to get themselves pruned for work the next day' i'd be less than impressed and think what a rude inconsideate person. It costs nothing to have some manners, except on occasion, such as a one off bloody wedding, may cost a tiny bit of effort or putting yourself out a little. Hardly rocket science is it.
All those saying well she wanted to go, and it was running an hour late, so she is fine to just do what she wants, must have zero understanding of social graces. Sometimes you need to be a little less selfish and actually, you know, consider other people above your 'wants' ffs.