It probably just felt like a fever, outside of him begin allergic.
Like others have said, start low. Even for naive users there is a large difference in how tolerant one is naturally to opiates. I know a guy that pukes if he takes 5mg of Oxycodone, where I would barely feel that even months after I hadn't used any.
I won't go into when and where to use them. For me it started as a social thing and ended up as a "sit at home by myself" thing. Just know that all opiates are fire and if you're playing with them. Play too often and don't take the proper precautions and you'll get burned by them sooner or later.