I'm a bit surprised too, but honestly I am a total wattage weenie. I vape things usually FAR lower than that, although I will say I was able to get an AOI kit that had lowest wattage setting of 30 watts, (on a 0.8 coil) to kind of work for me, but it required some breaking in and finding the right driptip, to limit the vape, so that doesn't sound OUTSIDE the realm of possibility to me. I mean, I'd imagine you could go higher with a 1.0 ohm coil and still get a good vape if it's designed for that.
The question would be I guess, does your wife vape those coils at lower than the recommended wattage (which is fine). If so, well, your Istick 20 watt mod may WORK, (and there isn't anything wrong with going lower than the recommended). I guess you could try it, and if she's not getting a satisfying vape below 20, then look at either a different topper, or a different mod that can provide more wattage and etc.
Best of luck
Anna