It sounds like a problem with circulation to me, but she really needs to try to find another doctor to go to that can be more helpful! if her feet aren't getting enough blood, or circulating the blood back out, it could cause the pain and swelling. no chance she is diabetic is there? diabets offten have hard times with ciculation and with healing from cuts or infections.(or a surgery) it could be dangerous....in your mom's situation though? find a good doctor, who can answer questions, and take her seriously.
Has she called the doc that did the surgery? Soryy I am 36 weeks pg and can hardly remember my own phone number so I am of no GOOG use right now, but with the hip or knee surgery (I don't remember which one) there can be problems with the foot dropping. I would call a ortho doc and try to find out, general docs. (I hate to say it) but are general and don't know too much about the bones. Good Luck.
I had knee surgery years ago. You would be surprised how much you start to favor the injured knee and it causes you to walk differently. It affected my hip and foot and I actually required a cortisone injection to relieve those symptoms. I think I had foot/hip pain for about 6 months after my knee surgery.
Did she actually have a knee replacement or invasive surgery?
I do know of someone who had a knee replacement and that foot bothered them really bad. It stayed swollen for months and caused lots of pain. We didn't understand why surgery to the knee was causing such problems in the foot, but the Doctor just kind of acted like it was no big deal and kept saying it would go away. And a couple of years later it is better.