On the grand scale of things, the surgery is pretty simple. It's done laparoscopically, so she'll be looking at a couple of very small incisions - much faster healing than the old style with a longer incision. However, it's still invasive and there can be soreness from the prior inflammation. If they're not operating immediately, it hasn't burst and it's not as much of an emergency. That's the good news. She'll need to take it easy but they'll have her up and moving pretty soon.
The thing is, she's at that age where she's an adult and probably wants to handle it herself - BUT she'd really like someone there. You know the 20-somethings. She's also probably not experienced in navigating the medical world, so an experienced adult can be helpful.
If you were just there and had a great time, you know you are welcome. If your ex isn't going and if her bio mom isn't around (I seem to remember she's not in the picture - is that right?), then I think I'd go. You have your ex to leave your younger boys with, so he can get them to school and so on. If you need an "excuse," say she'll need help dressing and showering/shampooing, and that's better for a woman to do. I know your ex is a jerk, but he might actually be relieved if someone went who knows what to do. If he's not, so what? She's an adult and he can't tell her who she's allowed to see. Meantime, you're teaching your boys what people should do when someone is sick or has surgery.