I would not get her high nicotine if she has lung problems. Try 10-12mg and 16-18mg eliquid. And you can get fresh foam stuffing for the Riva 510 cartridges at strictlyejuice.com (search for foam) and that will last longer than the stuffing they come with and is easy to rinse and pat dry with paper towel. For cheap juice see Madvapes. Buy a 510 drip tip for testing juices or she may want to vape that way, without the cartridge.
If you get the Riva 510 kit - I recommend Boge 3 ohm cartomizers, at least to see if she's capable of keeping a cartomizer damp. A 510 delrin or plastic drip tip can be used in place of the cartomizer endcap so it's easy to remove for adding juice. I can get 7-10 days use out of a Boge carto so that is a cost of $2 for that whole time (very little added expense to use cartomizers for favorite juices). Atomizers can get dirty or fail, cartridges can dump 1/2 their juice. Cartomizers have a bit of a learning curve but deliver a more consistent vape. Fill one well (hold tissue on the threaded end hole and put in 18-24 drops). Clean it up. Then lay on different sides for 15-20 minutes to make sure the liquid is distributed and all stuffing is damp. While vaping, if the vapor lets light or dry or tastes off or the carto is getting abnormally warm, add juice. I also look at the stuffing - if the top of the stuffing is white/light then you can try adding juice and see how many drops it wants. If those work out, the BOge 2 ohm ones are hotter (though they use the battery charge up faster)
Never screw anything in or on anything further than needed for it to function. (attys, cartos, battery on charger) That will avoid pressing the battery center post in too far (fixing that can damage or kill a battery)
----
if you or she are dead set against a fat battery model, then instead at Liberty Flights look at Joye 510 MANUAL kits (not auto - auto is a good way to kill batteries, and manuals make it easier to get a good hit). Then as above, get the drip tip and the Boge 3 ohm cartos. On the slim 510 you cannot use the hotter 2 ohm cartos. This will have very short battery life of a couple of hours.
Or look at FreedomSMokeUSA.com or elsewhere at the Joye 510 mega xl manual battery kits. Those have 60-70% more battery charge. (and get the drip tip and some Boge 3 ohm cartomizers)
For all, get eliquid. You can always add the fat battery Riva 510 system later if you want. it can use all the attachments the slim 510 uses but uses its own charger cable.