The S-isomer is roughly 4-5 times as potent as a dissociative as the R-isomer, meaning esketamine is going to be almost twice as potent per mg than regular (racemic, RS-) ketamine. If the OP is so hung up about being able to rail it in lines, they (or their gf) are going to have even more of a problem with esketamine. Also, I am pretty sure that even esketamine will still make you nauseous if you don't take it on an empty stomach.
I fully agree about carefully measuring the doses though (a decent scale like the Gemini-20 can be had for 30$, and it is sufficiently accurate for weighing out doses of around ~20mg). At the very least, weighing it out could give someone a better idea about how much material actually is in their average "lines" and "bumps".
OP, I believe you when you're saying you're not pressuring your partner to take more ketamine than they can handle, but peer pressure can often happen on a subconscious and unintentional level. Rather than diluting her stuff, just tell her to do smaller lines/bumps, and wait longer before re-dosing, reassuring her that this is not a competition, and that everyone's "ideal" dosage is different. After all, people do not water down whiskey by a factor of 1:10 in order to chug it from a beer mug; they just use shot glasses instead.
Ketamine is not an excessively potent compound; not that I'd ever argue in favor of "eyeballing" dosages, but this isn't PCP, where a mere 10 mg would be strong enough to completely floor the average person. It is thus not necessary to further cut the drug; just do smaller doses.
In general, it is better to get beyond the idea that doses of drugs are to be measured in extremely vague arbitrary units like "lines" and "bumps", and instead try to get a feeling for measuring doses in milligrams (even if its only a rough approximate). One cannot expect to have everything come in a concentration where a 100-pound-woman with no tolerance can snort similar-sized lines to a 220 pound male with a certain degree of tolerance.