I think if comfort is her #1
priority, she would be best served by sitting on and test-riding as many bikes as possible to determine what is comfortable for her.
And although this is probably the answer you didn't want to hear, it would be best to do this at your LBS because they will know how to adjust the bike properly for her.
She might consider telling the employees she is looking for a "comfort bike" or "comfort hybrid". Each LBS will have its own combination of brands, but pretty much all the major brands have something in that class, mostly in your price range, that will have a cushier seat and a more upright riding position. A lot of people find that the most comfort-friendly combination.
Probably the friendliest seat I've used was on my friend's KHS Green. It is wide and soft.