I don't know much about plucking but I'll do what I can. From what you've said, this is apparently emotional plucking (missing your mom) and not a medical problem in the bird. Perhaps some new toys would help cheer him up? Maybe give him some interesting new adventures, like putting him in a travel cage and taking him for a ride in the car.
It might help if you could arrange for him to hear your Mom's voice. Get her on the phone and hold it up so the bird can hear her talking, or have her record something that you can play for the bird over and over.
There's a Yahoo group for pluckers at
http://pets.groups.yahoo.com/group/pickinparrots/ It looks like it's not as active as it used to be, but there's probably still good advice to be had over there.
Cancer is basically an internal malfunction, and as far as I know birds don't have any effect on any kind of cancer. But cancer treatment can mess up your immune system so it's probably best if somebody else does all the cage cleaning. Although I had chemo last year and kept on doing all the bird maintenance without any problems.
It's a good idea for her to ask her doctor about this. I hope that everything goes well with her treatment and she beats this problem!
