Ah hunnybun, that's terrible! Chiming in with other posters and a little something new to add.
Go to the store, speak to the manager about this awful incident and get a full refund. That bitch MA
doesn't deserve to have her sales quota boosted by you. Complain to MAC corporate as well.
Take the $400 you spent and invest the majority of it on great skincare products. When I was in my teens, I had acne [not really bad but it was bad enough]. I tried to combat it with cheap drugstore products. Think about how much better off she'll be and how much she'll actually learn to enjoy the process of cleansing, toning, moisturizing, etc. with REAL skincare products.
Murad acne treatment complex or Paula's Choice is a good line to start off with. If you purchase directly from either Murad or Sephora you'll get your $$ bad if it doesn't work. Dept store lines like Clinique or Estee Lauder don't really work that well for acne IMO, way way too harsh.
Take the rest of the $$ and get her some great mineral makeup. MAC eyeshadows and lipglosses are great, but stay completely away from MAC skincare and foundations. I tried a couple samples of MAC foundation and they turned yellow, caused my skin to itch and when I look at the ingredients, YUCH. I won't touch 'em with a 10 foot pole. IMHO, MAC foundations are TERRIBLE for oily, acne-prone skin.
Instead, use liquid or powder MMU foundation, concealer and finishing powders. One touch of an uber soft kabuki brush and she'll be hooked.
Stay away from Bare Escentuals, however. The bismuth causes further irritation and an ugly, metallic sheen. I really wish I would have had mineral makeup when I was a teen. AT least the concealers and MMU won't cause further irritation and clogging and will help her face the day.
You teach her how to put on her makeup sweetie; if you post on this forum, you probably know more about it than most and you'll be able to mix the makeup lessons with love, kindness and consideration that MAC MA BITCH so totally lacked.
Please feel free to PM me for more info on mineral makeup and skincare routines, and give your niece a big hug!