I have Wisteria and have never had this problem. As far as I know they are not toxic so it is probably something else of maybe he has developed an allergy to it. It might be a good idea to run this by his doctor.

you can however become allergic to anything at any time in your life so gloves next time .

The RHS list Wisteria as a skin irritant - this is what they say
'Contact hazard
Irritant sap may cause a burning sensation and sometimes blistering of the skin; anyone can be affected if exposure is sufficient
Some plants contain chemicals called allergens. These do not affect everyone, but some individuals may acquire sensitivity to them, resulting in an allergic reaction
A very small number of plants have sap which renders the skin excessively sensitive to strong sunlight. Contact with the plant followed by exposure to sunlight results in very severe, localised sunburn with blistering and long-lasting skin discoloration'.
If it were me I would apply anti histamine cream this time to help reduce the discomfort and get some one else to prune it or prune when the sap is not rising in future while wearing protective clothing.
I have a problem with several plants in my garden irritating my skin Feverfew is one and Euphobia another. .

As others say, yes, it can irritate - if he's only pruned in winter before, he wouldn't have known because the sap's not present in winter. He'll need to cover his arms and hands next time, in summer.