I can identify with the arthritis problem. I doubt she will want to be swinging the leg over the seat to get on or off so definitely recommend the most angled top bar.

If she goes with a step-through she will probably be limited with very heavy bikes at the local bike stores. Even though she won't be swinging it up on a bus rack like I do, the heavy bikes just feel clunky.

3 miles doesn't sound like a lot unless you are a new biker who may be a little hesitant about anything but very flat ground. My arhtritis flared today something terrible and it was all I could do to finish 4 miles. Consequently, though she may not think she needs a lot of gears, I think it is very important to get her a bike with really easy granny gears for those terrible days.

I can recommend the PereformanceBike "Soft Tail" Saddle. It's gel, a bit wider but it's definitely not a tractor seat and seems to work perfectly in keeping down the jarring of spinal arthritis pain.

Running around to bike stores is confusing and exhausting for beginners. Have her do a little internet window-shopping.

Here are some ideas:

Trek Women's 7000

'09 Trek Women's Navigator 3.0 This one is heavier than the others but comfortable.

2008 K2 T:Nine Skye Ladies Comfort Bike

GT Transeo

Is there any way you can visit her and take her bike shopping? She would love that!