People do generally line up for princesses well before the 9 AM start time, when they can. I think we had a nice short line on Friday in part because the PFF opened at 9 AM, the same as the park, so no one was able to line up for PFF before it opened. Some of the "regulars" may have been figuring to show up at 9:30 to get in line for the PFF opening an hour later than the park, but PFF was already open. And of course many families with little kids have trouble getting them to the park right at opening. And people were going for WoC FPs (though not as many as I'd thought!) and the parks weren't actually that busy that day.
But in our particular case if princesses had been the top priority, then we would have wanted to straight from the gates to PFF, and given that we were all adults, there's a good chance we would have been close to first in line.
IF Snow White, Aurora, and Cinderella are indeed the first three who appear at 9 AM (I don't know if that was the case for us or not, since we didn't meet princesses until sometime after 10 AM), then your son and his girlfriend might actually get duplicate princesses if you're in line at 9 AM, as it looks like those three also do the first coronation ceremony. I'm not sure if there's any solution to that problem, though. From my very limited experience it seems like you'd want to time it to hit the princesses themselves at about 10 AM (unless the coronation is a priority, in which case, maybe 9:50 AM). But obviously you can't be certain that will work!
The storytime was cute but nothing particularly special. It was just one of the princesses (Aurora in our case) sitting on the throne-chair thing and telling a story, with very minor interaction from the kids. I wouldn't suggest they stay for that. If your DS's girlfriend likes Tiana, they'd be better served trying to catch her show in New Orleans Square sometime when it's playing, IMO.
I DO agree with Raindown that your DS will be a hero for waiting at all, anywhere.
