Hello,
Skin rash and itching is a common side effect of Thiamzole and this itchy skin rash that your mother is having can be due to hives as a result of the side effect of this drug. For such hives, first of all the medication needs to be changed to some other drug. Then the primary treatment is by oral antihistamines. The most potent antihistamine, however, is probably the anti-depressant doxepin. You can apply an anti-itch cream or lotion to the affected area. Oral antihistamine, such as Benadryl can also be used. Your mother should take a comfortably cool bath and wear smooth-textured cotton clothing.
If the symptoms persist then get an evaluation done from a dermatologist as topical and oral steroids may be needed.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.