Difficult to tell from the pic... and without knowing if the horse could have been in contact with anything that could've caused a reaction of some kind. Also, are there any similar patches on the horse or any of his other field-mates (if he has any)?
If I wasn't worried about it being something sinister, I'd possibly put something like Aloe Vera Gelly on it for a few days, and if no improvement see if the vet can 'pop in' or see if they can advise on the phone. But if your friend is worried and it's got worse, then maybe chat to the vet now.