Good idea. I encountered a similar, very similar situation with my big sister. She's not that big @ 5'7 + I guess but she's an "agressive" or more aptly called a strong/ guy-like rider. A few bikes she'd borrowed and bought are just plain too short. She hates drop bars and will only tolerate a step thru. Not nec. to change your plans, I didn't, I just ened-up with a number of "wrong bikes", more than I care to admit, too many female specific bikes. Female spec. bikes are shorter, always. At 5"10", your lady's tall.
Pricier models typically are offered in the not so small sizes. Newer, upright Hybrids are short, short.

Mixte frames are all the rage. Motobecane, Peugeot are pricey, the pre-1979s or some Pugs. even later have that funky thread think goin' on, I'd be hard pressed to rec. 'em. In their defence though, these brands are less shorty types. You need a 19 to 21" ((48cm to 53 or so) virtual top tube length. Meas. center of seat tube to center of head tube. Hey... I'm not speaking out of school here, it's by your own admission that such a size is warranted. A guy would need one notch up, perhaps. Guys ALWAYS have longer arms and our bodies bend lower down on our torso. I've fit 5'9" gals as well. Many fem.frames are 49cm(19") CORE size which is seat tube length, though not of concern as such, being a step-thru, is the nominal size of bikes. Youi need to shoot for closer to a 21" as mixed and lady's frames, other step thrus were meas. inches.

I looked in e-bay, two that I spotted; a Miyata 10, a lesser Miyata and a Schwinn. I've Nishikis, B.Stones as well as the first two brands. Don't dismiss Japanese bikes. IMO, I wouldn't dismiss other step-thrus either. True Mixte frames were offered on and off. Availabilty will dictate choices so there's no point in my giving a long list. As for Schwinns, NON mixte; World Sports and up are good. Sprints OK but cheap.