If it's "extensive and unpleasant" she should see a doctor or at least the local minor injuries unit.

I get minor allergic reaction to blackberries, in the form of large areas of intense itching, for which antihistamine cream gives some relief (although mild, it's still enough for my to have replaced my berries by thornless ones which don't irritate the skin and allow the thing I'm allergic to to get in so easily) And to euphorbia sap, for which I use a tiny bit of hydrocortisone which seems to ease the blistering - ask your pharmacist before using hydrocortisone for this.

Remember that some allergies can be sun-sensitive - ie they can recur without further contact with the plant whenever the affected area is exposed to sun. Rue is like this, I believe, and I'm beginning to think euphorbia is too.

And is she sure it's the white bryony, not the redcurrants? Allergies can be acquired - the blackberry allergy is something I acquired after 12 years of growing, picking and pruning.

In future, cover up before picking - long sleeves, possibly thin surgical gloves (the skin on the hands is tougher than the inside arm skin), and a pre-emptive barrier of handcream can help against a mild allergy.