You need to use less nicotine to get the ultralight effect. An ultralight smoker may find 10-12mg sufficient and certainly enough TH on a decent ecig.
Juicewise - I will recommend an odd choice - Backwoods Brew Casablanca. I was a Pall Mall lights smoker when I quit to vape (ugh I hated Pall Mall Lights but they were cheap) but my true love in cigarettes were Capri Ultralights - my boss called them Barbie cigarettes - they were long and skinny and mellow with a noticeable amount of cocoa. Note - most US cigarettes contain cocoa as well as something sweet like honey or sugar - like Marlboros, Parliaments, Pall Mall, Capri and others.
Casablanca is a complex Turkish blend with a bit of cocoa. It is 70pg/30vg so if you use it in stuffing-type cartomizers I might thin it just a bit by adding a small amount of distilled water. Or if you want more cocoa, try adding a bit of Vapor4Life WOWbacco (a cocoa tobacco) which is a very thin juice.
Casablanca was my first all day vape and it made me go ahh like those Capri Ultralights did. Just something about the complex aroma. I recommend you add it to your short list of juices she should try. I literally tried dozens of tobacco juice flavors before trying Casablanca and Casablanca was the first that really hit the spot for me.