Are flying V guitars good?

No. There are a few reasons. The Flying V is huge, for one. Most likely too big for someone that age. It also has a tendency to slide when sitting down. This would be fine, but most of the time she will be sitting in front of a computer, or practicing sitting. Playing with a strap is more difficult than it would seem at first. A quick note on playing with a strap: The Flying V is a heavy guitar, (thus the wide tonal range) and I don't think a 12 year old boy could keep it up for too long. I haven't even seen too many older girls playing heavy guitars. If she still wants a V, you would be better to take a look at the Jackson Rhoads, or King V series. They would be much lighter. Before buying anything though, go to a music store. Try a few instruments sitting and standing, and make sure they are comfortable to play. Whatever you do, don't get a strat shape. Everyone has one. People who can't play don't tend to have anything else, so at least she can look like she knows what she's doing!

When you're a beginner, any guitar that you can get your hands on is good.

Nothing wrong with Flying V guitars. The principle is pretty much the same with electric guitars, whether it's a flying V or a Gibson SG, or a Fender Strat or Telecaster, or Ibanez, or whatever...

The shape of the body is no big deal... the different types of guitars will be set up with different pickups, and that is what fundamentally defines their sound. All of them will sit on your lap just fine when playing, no significant difference there. A flying V will typically have humbucker pickups, which results in a fat sound suitable for heavy rock. Damn good thing too, there are too many sissies and wooses writing ballads and not enough kickass RAWK in the world, in my opinion...

In answer to your question, a flying V is absolutely fine for learning guitar. You will be able to apply what you learn on that guitar to ANY style of guitar. I learned on a Fender Strat... and had no problem doing exactly the same thing on different guitars.

The only downside with a flying V is that they are not made so you can sit comfortably and play them. Most guitars have a waist which you can rest on your leg to play them in a sitting position. Apart from that they are just the same as any other electric guitar. Flying V is just the body shape.

Whether a guitar sounds good or plays well depends on the hardware and the build quality, not the body shape.

No, they are horrible for playing in a seated position. But, in my opinion playing guitar in the seated position as a beginner is not a great idea. I do sometimes practice scales while watching TV, but in general you want to practice standing and playing with a strap because in the real world, whether playing a gig or just jamming with friends, you will be standing.

When I started playing I always practiced while seated and when I tried to play standing it was like I had to re-learn everything. But again to answer your question, the V is almost impossible to play comfortably while sitting.