I am not convinced that a flat-foot bike is a great idea. They sit lower to the ground, but actually fall over faster because of that.... for the same reason that a matchstick will fall over faster than a telephone pole will.
Search Google for "Day 6" bikes.
I have a RANS crank-forward bike (someone else suggested). They are expensive but the frame kits aren't bad (just find a cheap bike for donor parts). I think it is a more-aggressive ride than someone elderly would want though, because the seat has no seat-back. You must pull forward with your arms to pedal.
If there's any interest in trikes, then you're pretty much limited to 3-speeds if you want an IGH off-the-shelf. The $1500 Rohloff hub is adaptable for trike use, but the 7- and 8-speeds from the other companies aren't easily useful. They may be adaptable
, but probably not without machine tools--another cog has to be attached to the left spoke flange.
There are "trike conversion kits" on lowrider sites for converting a bicycle rear-end into a trike and they're not expensive ($150) but these all only come with a single-speed rear hub.
You also might take a look at the Biria Easy Boarding bikes--a few different models, there is a 3-speed IGH ($~450) and 8-speed IGH (~$750).
These are not "flat foot" bikes, but they are easy to get on and off.