I myself have severe foot pain due to damage from rheumatoid arthritis.  I also have fallen archs on both feet due to damage and uncontrollable swelling.  That being said I do know a couple of things that may help.  I was once told by my rheumatologist that crocs are wonderful for swollen feet.  The are made a little wider and don't fit too snug on your foot.  Orthopedic shoes also seem to help many and are available in many different styles now.  As far as inserts that help, I always use the memory foam inserts which shape perfectly to your foot and provide a lot of cushioning when you walk.  I use these all the time and they are wonderful!  I have to wear slippers a great deal of the time especially in the morning and at night when my feet hurt the worse.  I use the slip on memory foam slippers and then add a memory foam insert for extra cushioning.  Now, as far as the pain itself I have found something that has done wonders for my painful feet.  A paraffin bath does wonders for both painful feet and painful hands.  It works so well that sometime I just want to use it all day long.  I hope this helps, and I hope your mother in law finds some relief from her painful feet soon.  Good luck and best wishes!