Think about fitting too...
High or low waist? Very low waist may be more sexy, but only if she feels confident and comfortable in it. Very high waist may be very flattering, especially with thick quality fabric that pulls in the front.
If you buy much too small size, the risk of camel toe increases.
Very cheap and thin fabric will show the panties through when squatting (if she wears anything inside)
Scrunch bootie is sexy but only if she's confident in it. Patterned fabric hide the scrunch look more than plain fabrics, but still pulls it in and highlights the butt cheeks.
Pocket on the thigh may be useful to keep mobile phone.
Good luck, in the end I'm sure she'll be happy whatever you buy.