She may have contact dermatitis,which can really be a problem. I get and have gotten severe bouts with it,and was seeing a dermatologist for it,after all the OTC things only partially worked.
One presciption product I had was a gel,not greasy or oily,called, Diapolene (sp?) no,it has nothing to do with diaper rash,I had hands that were cracked,itchy so bad I literally would scrath to they bled. Or I'd get all little fluid filled teeny bumps thst itched,and then ooze.
I had one summer where I was in constant treatment with various topicals. I can get a reaction from some handsoaps while some do not bother my skin at all, I react to a brand called "Softsoap" terribly, the most. Another thing too, if you use rubber gloves for cleaning be sure to use a cotton glove liner,it's a thin 100% cotton glove(s) made for that.
See, that use to really be a problem,when I'd be stripping antique furniture,my own perspiration of my hands would set up a ideal environment for dermatitis,using the glove liners helped a lot.
I can sympathize with anyone dealing with this,even some moisturizers can react,making the problem worse instead of soothing.
The best thing for me when my hands are really dry and starting to crack,is get a jar of Cetaphil cream,at night cover you hands with it and then put a white cotton glove on it,some stores sell "sleeping" gloves and even "little peds" for the feet,designed for the same purpose.
The best thing is to see a dermatologist,to help rule out the cause and then prescribe proper treatment.
It most likely has nothing to do with her diet,but things we all come in contact daily.
I get the same reaction if I handle any juniper plants,or tomato branches,besides certain soaps.
Hope things clear up for her soon.
Take care,
DE