This is the only place you can ask as a new member and actually the best place to ask.
What is a lot. How many packs a day and what kind of cigarettes? Also how technically competant is she and does she have any special needs for battery life or is she near AC power most of the time?
The problem is, heavy smokers often need the fat battery models with 6-8 hour battery charge. Especially if they smoke full strength cigs or need a hard hot drag. The fat battery models Ego and Riva 510 can use harder hitting low resistance parts. You cannot use those parts on slim ecigs. but she may be resistant to something that isn't slim and cylindrical?
Forget about getting anything the same size and diameter as a standard marlboro - not strong enough for anything but an ultralights smoker and the carts are extremely tiny and need juice every few cig's worth of drags.
If she is near AC most of the time for charging and swapping batteries, look at the two most popular slim models and I recommend the mega batteries on the smaller one (510). I recommend manual batteries on everything. Autos are much easier to kill with a juice leak and can turn on accidentally from wind or vibration. The manuals, though you need to press a button, are actually less work IMO - you don't have to drag hard to make them work but instead you hold the button down from slightly before your drag until slightly before the end of the drag.
KR808D-1 (at LiteCigUSA.net, VaporLeaf.com, and in colors and prefilled cartomizers available at Vapor4Life.com). This is a 2-piece, about 120mm long with the standard batteries and a light to moderate vaper can get 3-5 hours out of a standard battery, a heavy vaper may get under 3 hours.
Joye 510 (see Cignot.com. LiteCigUSA.net) - the standard batteries only last 2-3 hours for a light to moderate smoker and might only last her 1-1.5 hours. The Mega xl batteries make it 138mm long as a 3-piece (it comes as a 3-piece) or I recommend you use 510 with blank cartomizers and with mega batteries and a standard carto it would be 128mm and have life simialr to standard KR808D-1
For 6-8+ hour battery life and ability to use the harder-hitting parts, you need to look at Joye Ego or Riva 510 for the 510 line