I recommend either KR808D-1 (also sold in colors at Vapor4Life.com) or Joye 510 with mega xl batteries for a 3-piece (KR8 standard batteries and Joye 510 mega xl batteries are both 280mah of charge, or about 3-5 hours, less if vaped heavily). PCC charging cases are available for both models for charging on the go.
These are the two most recommended slim ecigs and the most supported with accessories. I vape both of them.
I recommend manual batteries - more sealed against getting killed by juice leaks, won't turn on by accident from wind or vibration, and I think they are less work and make it easier to get consistent drags
Some good hardware vendors:
LiteCigUSA.net
Cignot.com
Liberty-flights.com
Vapor4Life.com (for colored KR808D-1 and prefilled cartomizers)
Buy some eliquid at the same time and then order more at an actual juice vendor after you get the kit.
The KR808D-1/Vapor4Life has lots of prefilled cartomizers but a carto needs to have jucie added when needed so it will last for days, not part of a day. If you run them dry you could singe the stuffing and ruin the flavor. You need to add juice to a cartomizer much less often than you need to add juice to the cart of a 3-piece. Cartomizers are disposable though - keep one damp and you may get 4-7 days or more use out of it. Some people are successful with cleaning them after that, some are not.
Joye 510 comes as a 3-piece - uses cartridges (aka carts) and atomizer and needs to be topped off more frequently with some eliquid. But you can get a variety of blank cartomizers for it and fill them yourself, using it as a 2-piece. That's how I mostly use my 510 - with cartomizers.
And if she has her eye on something fancy on the web - beware. There are a bunch of ripoff vendors on the web, usually the glitziest sites with live help and "testimonials". If a ecig site charges more than $20 for a battery or does not sell blank carts or cartomizers and juice, do not buy from them. To me that means they are just out for your money and want to lock you in to buying from them. We get people here every day showing up hoping we can identify what model they actually have so they can get supplies elsewhere. Sometimes they are unidentifiable or poorly supported models only use by other ripoff rebranders.