I have styled some outfits for her.
The first blue dress is from Warehouse and is on sale for £50 from £75. It will work well with a statement necklace. I would stay away from black as it's the summer, and the prom is a time to stand out.
The 2nd dress is from New Look and is now on sale for £18 from £28. I actually have this dress and it looks so AMAZING on, it looks very expensive despite the price. This will work well on a nice summery evening. You can tone down the accessories if you wish or keep it playful.
The 3rd dress is from Warehouse and is now on sale for £40 from £75. It's so cute teamed with toned down accessories and statement earrings.
The 4th dress is also from Warehouse, they seem to have a nice range. Priced at £50 from £70. This purple dress screams "cuteness" I have opted for gold shoes and classy clutch bag with enough detail and colour in the earrings to bring the dress to a complete level.
I have also added 4 more dress suggestions that I think will work well for a prom too:
Click to enlarge!I hope this helped, if you are still stuck go downstairs to the concessions dept at Topshop, they have masses of beautiful dresses!