There's not one type of 'mixed race' hair, so I'm assuming you mean Afro hair, or a slightly looser texture. What type of braids is she getting? Cornrows or individual plaits?
Shoulder length isn't too short, but extensions will usually be added in to bulk the hair out, increase length and longevity of the style, and it helps with styling them (wearing a bun, or a ponytail, half up, half down etc)
Keeping them neat varies. You wouldn't typically wash your hair once they're in, to preserve you can buy sprays specifically designed for braids (found in Afro-Caribbean hair shops) to keep the hair moisturised, and some people wear a scarf or a bonnet to sleep (I never have). I re-do mine about every 4 weeks.
Ok I took a look, she has beautiful hair! Braids are usually seen as a protective style, so I'm guessing as in my previous post they want to add extensions just to keep the hair neater while it's in braids and to increase the length and volume. It's a optional thing, she doesn't have to have any hair added in, her hair is a pretty good length as it is - but it wouldn't stay neat for as long without.
My main tips are really to avoid getting her hair wet, make sure they are not too tight and pulling on her scalp, avoid scratching her head, and a good stylist should be able to recommend hair products to use to keep them neat. As said in my earlier post, you can buy 'braid sheen sprays' to keep the hair moisturised or alternatively use a leave in conditioner spray.