Anything smaller than a Joye 510 is not going to satisfy anyone very long.
Joye 510 with cartomizers maybe? That is the smallest thing I would recommend. Cartomizer makes it shorter and less hassle than cart and atty. Downside is standard 510 battery only lasts 1.5-3 hours, less if vaped a lot. But you can get a PCC (personal charging case) and use AC to USB chargers when near AC. They have longer mega batteries (I get 3-5 hours) that may make the battery+carto over 125mm but doesn't look that bad on a female (these need a different PCC). Looks nice and slim in black (has a blue led in blank I think)
I am using these blank cartomizers
510 Blank Cartomizers
And if she wants a soft hit then don't get 100% PG juices. What kind of cigs does she smoke? if full strength then she probably does need 24mg, lights try 16-18mg, ultralights try 10-12mg (and some people need more, and some who vape a lot need less)
PG lets through the most TH harshness of the nicotine and the most flavor. VG dulls TH and flavor too somewhat but makes more vapor. I like 70pg/30vg. Backwoods Brew has nice complex tobaccos and I also use FreedomSmokeUSA tobaccos. At FSUA you can also try 50/50 juices - less TH and more vapor.
Then there is KR808D-1 also sold by Vapor4Life - lots of prefilled cartos you can refill for several days (I am on day 6 with one I am using on my KR8). It's a great unit but fatter than 510. Its standard batteries last as long as 510 megas
I do not recommend auto batteries on either. Especially the 510. Open hole on the end of the battery - if you get juice in it you can kill or disable the battery. Not really safe to be refilling cartos and using on an auto battery. Most people on this forum recommend manual batteries. They let you control your hit better, more consistent, less work (no extra sucking to warm up the atty) and less prone to being killed by juice leaks.