Lotta good info here, but I'll chime in with my $0.02 (and likely a bit more).
Proper fit is HUGE. And by this, I don't just mean frame size, it also has to do with the lengths of the different tubes of the frame's & the bike's geometry (frame angles). This is because 2 folks of the same height can have completely different builds. Like me, I'm 6' & have a LONG torso, so when looking at bikes, I want a longer top tube, & often enough, a long stem as well.
Also, as I'm a big guy, so I run with wider than stock/average handlebars. This is because I have a wide chest, & the wider bars let me open up my chest more when riding, so that I can get closer to fully expanding my lungs easily... and thus, ride faster. I was Shocked, & quite happily Amazed when I first made the switch. 3cm wider bars made a HUGE difference.
A long top tube does the same for me also (letting me stretch out, & fully expand my lungs), in addition to it's allowing me to get my torso more horizontal & aerodynamic when riding.
The same logic applies fit wise to mating a bike to the various key body parts of a rider's specific physical build.
On the frame materials thing, again, some of it goes to fitting the material to the rider. Me, I prefer aluminum & stiff frames. The why being, again, I'm big. Not a giant per say, nor fat, but big by cycling standards @ 6' & 210lbs. Which is about 1.5x the norm for serious & or pro riders. So my Cannondale helps to soak up the bumps for me, where as it'd jar a fly-weight rider. PLUS, it lets transmit the extra torque of my 1.5x cyclist body to the rear wheel, instead much of my pedaling energy & torque being absorbed by the frame flexing back & forth whenever I really stand on the pedals. This used to happen when I rode a top quality, steel, Bianchi. Ugh! The latter was a great bike, but comparatively was like a piece of well cooked spaghetti compared to my current bike, & some other, stiffer, rides.
Also, how big of an issue are pot holes & the like in your AO? I ask as some materials handle crappy roads better than others in the long run. Ditto on the type of wheels which you ride, & their spoke number + lacing pattern.
Regardless of what she/you guys choose, much like in shooting, there'll be a lot of extra accessories to buy, which you may not have figured on right off. Likely the first would be a spare set of wheels or three. Say a set or two to train on, & or beat on, depending on how you view it. And a racing set or two.
Some of them will likely be of different materials & designs. Especially the rims portion; road bike style, semi-aero, full aero. And of different weights & durability. The super svelte ones being for competition, but typically more delicate...
I mention wheels, as aside from the frame set, they're the biggest weight factor on the bike. Particularly, as they're rotating mass. So every extra gram in the wheels + tires is a BIG thing every time you go to accelerate. Much as is the case when choosing running or hiking footwear. It's a heck of a lot easier to run 6 minute miles in Nike's than in Danner's.
You CAN get a lot of Spectacular deals in used bikes, or when a store's clearing out last year's or 2012's models & the like. Particularly if you've done your homework on what you're after, & have a cheat sheet of what your top 3 choices are in terms of each component/part of the bike. Including the frame set (see the geometry bit above).
And like building a rifle, if you've got a touch of extra coin, you can for example, pick up a low mileage bike which has a frame that was all the rage 2 years ago, but has what are now "so, so" components. Then add & upgrade things as you go along/find out what you like. Which is where, once she gets into riding in a club & or competitively, she'll get lots of chances to try out other folks gear, including the way high end stuff, & get a sense of what she likes & what she doesn't.
HTH, & good luck both shopping, + riding & competing.
PS: Sometimes it helps with the bottom line $ wise, if you find a bike that fits the bill, in a shop. Then you also buy a helmet, shoes, shorts, lights, repair kit, etc... all at the same place, then you can quietly haggle with them for a package deal. Especially if they know you'll be a repeat customer, particularly for accessories & upgrades. As well as that you'll spread good things about them in the cycling/tri community via word of mouth.
