For brand new, another vote for a Brooklyn. I got a Clem L for the wife but it was still too big for her, so I got her the Willow 3 instead. Geometry and ride are very Riv like, though not as "sharp" so I replaced some parts to tighten up the ride. Frame's not bad though to build on, and they're one of the few manufacturers of city bikes who use cromoly, at least in the Willow. (Careful with those entry level bikes as "steel" is not synonymous with cromoly.)
Still, if she fits a Riv, I'm with Joe and check out the used market. It may seem pricey but depreciation on a well kept step thru is not bad and I'm sure the net "loss" of owning it would be worth the expense. Also, step thrus have a wide PBH range for fit: that 50cm Cheviot in theory could tolerate her growing another 5" or so.
--Ed C.