I'm plus size. Top tips - really polish the grooming. Hair, nails, teeth. Making sure her face looks great - make up can be minimal but effective. Try going along to a make up counter such as Bobbi Brown for a few ideas - they will give her tips on a look that flatters and correct application. Less is definitely more.
Decent underwear will make a huge difference to how her clothes sit and appear - a good well fitting bra (avoid the double cleavage look!) and a pair of shape pants to wear for interviews (give a smooth line under clothes).
What is the expected dress code for the jobs she is trying to get? Is she pitching this right? Too formal or not formal enough?
Can she use the sales to invest in a nice pair of interview shoes (agree that a pair of wedges will be versatile) plus a good quality leather bag.
Other tips (realise they may be horribly obvious) - dial down piercings and visible tats. No underwear on display. No loud, chunky or punky jewellery (go for a simple pair of studs in ears and a watch). Top notch personal hygiene with understated/light fragrance. No orange self tan.
Don't forget basics like punctuality, preparation (questions to ask at interview), good posture and manners (no chewing gum, fiddling with phone etc).
Wishing her all the best.