anything you can do to stop his friend giving him a haircut? Nope...I'm afraid. My sow often gets her cage mates to visit her salon - much to my annoyance - although she hasn't done it for a few weeks, so maybe it was just a phase?
The only problem with applying sour tasting lotions etc to stop the barbering is this may discourage the barber and barbered guinea pigs from washing/grooming themselves - something you dont want if you have a long haired guinea pig.
Unfortunately it's something ive come to accept - unless it's causing the barbered pig great distress, it's something you'll have to put up with - ive watched my sow barber her cage mates and they all stand there allowing her to do it - so it's deffo not causing them any distress/pain and they appear not bothered by it at all.
Some say it's a dominance thing, others say its down to boredom or hunger - just make sure they have a constant supply of hay and plenty of chew toys - hopefully the new hair stylist will get bored and stop giving his new mate a haircut!