Great idea. Not only safe, but so much warmer than any other headgear. This is possibly one area where UK mail order might not be that cheap, and buying one in the resort better, partly due to supply and demand, but also due to the high postage cost of a return due to its bulky nature if you don't get the size right. Ski helmets aren't exactly common in the UK, whereas they are 'bread and butter' in a ski shop where it snows for almost half the year and this tends to keep prices down in the resort. Great brands to look for are Giro, Uvex, Smith, Alpine. She really does need to try a few side by side though. In most ski resorts there will be a number of shops and this also tends to keep prices down a little. You might therefore be surprised by how cheap stuff can actually be in resort. It's a bit different to bike helmets. I know I have a a sml/med 'Giro head', but in ski helmets its a whole different fit process. Possibly not the answer you wanted to hear, but as a regular skiier, I long assumed that resort ski shop prices were bound to be more expensive than 'back home' based on bike price experience, but it isn't the case in my experience of skiing mainly in Austria. Can't speak for the French resorts of course!
Styles are split in a similar way to mtb helmets (race/XC/freeride/downhill). Racy types which tend to be very close fitting and not that well ventilated (unless you are racing, steer clear – it's like wearing a full aero bike helmet on a £50 supermarket bike), 'dudey' types (bucket design and quite cool in snowboard circles) and standard helmets (normal looking, sometimes with peaks/visors).
Things to look for:
1) Some sort of removable padding/ear muffs should the weather get warm.
2) Venting as above (turn off and onable). The ski helmet has to work at both keeping you warm and cool should the sun come out. Because of the full coverage, they can quickly get too hot in the sun.
3) Full shell (some have exposed polystyrene and this tends to wear faster).
4) Most importantly good fit.
5) Goggle loops at the back (but these are pretty standard).
6) A clasp that you can do and undo in gloves blind. Some are simpler than others.
7) A good interface with your goggles. Not all goggles and helmets work well together (another reason you need to try before you buy).