It only costs about $500 to bootstrap a jewelry store on yahoo stores ($40/month) and about 30 skus from pandahall... I use them for my online retail exeriments. I buy lampwork glass pendants at $0.25 each and sell them online for $15 each with free shipping in US... COGS+shipping is about $5 so even if I have to replace a whole order I still make %33 gross profit. My COGS + shipping is high because I don't buy in bulk on shipping materials and I ship USPS with a tracking code.
I package orders for 30 minutes to 1 hour in the evening and drop them off at the post office on my way to work...
(My real gig is at the scale of amazon with a 200k sqft fulfillment center and 14K SKUs)
This is a relatively low risk white hat venture and it is very educational about all aspects of legitimate business. It has strong emphasis on sourcing quality products that sell and marketing strategies... if she likes fashion, then she'll probably really enjoy this kind of venture...
It takes about 10 months for SEO to kick in on this kind of thing so she'd have to do social and cause marketing to get it off the ground. But teenagers tend to be social and family and friends are quite supportive in this kind of thing so I'd imagine she'd do pretty well, and I don't even know her.
Good Luck!