My first thought is that all of these bikes will underperform doing centuries, etc. A flat bar does not offer enough hand positions for that amount of time on the bike. I would find something with normal road/drop bars that has an upright stem and position.

I went through a similar toss while picking a commuting cycle a few years back and test rode all three of those options. I ride an '06 Sirrus base model now. I'm also just getting into Tri and have been training with my friends on full roadie's and can keep up no problem. I liked the specialized b/c of the rigid road feel and it's quickness. I'm in downtown NYC so nimble and durable are two of the most important factors. At the same time i've done 50-70 mile rides on it.
I have made some upgrades over time but nothing beyond what you would upgrade on a racing setup: Saddle, Wheels, Stem, Pedals. I know there are levels of the sirrus as well, but they seem more like just bells and whistles. I've never had drivetrain issues or frame integrity concerns.

Admittedly, I think the best bang for the buck in a flat bar bike like this is a Felt Speed 33 it's basically the same price and a nicer package. I'd check that out too.

I'll pass this on to her and then I'll duck...don't kill the messenger! Just kidding (Hey N) :)

I think the possibility of this:

"They can also probably adjust the seatpost and handlebars such that when she's on the brakehoods she's in a much more upright position which will help ease the transition."

is a great suggestion and something that really hasn't been mentioned by the shops-to my knowledge.

+1 to what Tamara is saying. I recently helped a friend buy a hybrid/comfort bike, but she was convinced it was only for little trips around town and that she has no desire to do riding other than that. However, in the process, I did have her get on the Specialized Ruby (she was worried road bikes meant leaning to far forward) and after that she admitted it wasn't nearly as bad as she thought it would be. I'm sure the shop can help set up a road bike to be more upright for her, and she'll be thanking you after seeing how much easier they are to do distances on (my first metric century was on a hybrid -- I bought my first tri bike on my way home ;))

Go with a road bike with a height adjustable stem (Ritchie adjustable) to raise the bars closer to a hybrid level, ,, then if/when she develops as a rider she can come back down, although might need a shorter stem then

I was definitely impressed with the Specialized Ruby, one of their women's bikes.