I'm not quite sure how receiving oral would make his penis sore... I don't want to preach, your friend should strongly consider using condoms so as not to put himself at risk for catching anything, there are STD's which you can catch by having oral performed on you. I'm not sure of his situation with his g/f, whether it's long term, monogomous, or what, but condoms are a good idea unless she has been tested and is clean and never gets coldsores.
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Sore After Oral Sex 
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