Allergic reaction from hydromorphone / dillaudid

honestly for once, I have a question not for myself, but a friend at work.
Personally, after a couple of back surgeries, and a long time off work studying these thing, along with first hand knowledge, I know of the histamine reaction people have from some opiates.
Last night, he had what looked like a major flea attack on his arms, I don't what to venture to say where else, but he explained it was from his pain medication... it only took me one guess when he said "stronger than morphine" as to what.
I've had a bad case of itches before, but never 30+ visible red spots on the forearms. I advised him to take a benadryl with it when he took them, but I'm a bit worried that it's more than will be handled by an allergy pill.
This seems to be the only symptom of a bad reaction, but can one offer better advice to clear it up, or an alternative to a drug the strength of oral hydromorphone?