tinkybell, dont worry too much for your friend. like what dophin said, t-shirt n jeans also can. i call this SMART casual.
there will be hundreds of ppl there. there is more than just her dressing they look at, her height, weight, attitude etc. worry so much also no use.
just tell her to do her best and dont be too late or else the queue will be very long and she has to wait for a long time before her turn.
if she doesn't get in the 1st interview, take this as an experience and apply again at the next intake.