We went to the store last night and picked up a pair of Continental Contacts in 37-622 size, which I guess is basically a 700Cx37.
As an aside, I found
this Sheldon Brown page about ISO sizing, which makes a whole lot more sense to me than the 28x business on the tire labels. Now I also understand more about rim measurement. The rims are 622x18, which according to the page linked to, will take anything from 28mm width to something over 37mm.
After a midnight ride - harder to keep up with her for sure - she's pretty happy with the change.
Thanks you guys for all the help. I would have wasted my effort insisting on something that said "29" and not gotten the same results at all.