Depends on her style, but I have always been a HUGE fan of Narciso Rodriguez. The tailoring on those things are SO figure-flattering. The designs are always so simple and classic, but with some modern detailing. However, the dresses aren't really like ball gownish so I'm not sure if that's what you're looking for as far as evening gowns go. You could also always go for a Herve Leger bandage dress (again, not full length ball gown; knee length and definitely a 'sexier' look than Narciso's tailored and primmer look). These are skin tight but sexy as hell on the right body--and good for its design, it creates a good body. http://www.saksfifthavenue.com/main/...er=herve+leger
Both my suggestions run around $1500. Anyway, why don't you just go to a Saks or Neiman or whatever, find yourself an SA in the women's department and ask for her help?