A friend seriously thinks that someone has hacked into his site and adsense account and is scimming of a big proportion of his revenue - is this possible? Are there any cases of this happening that you know of?
A hacker could make it so that ads using his publisher ID appear, at least occasionally. If it's happening a lot, your friend should make sure the only publisher ID used by his ads is his.
Yes this is possible I would recommend your friend take a look in his code and on his server to see if there is anything there that shouldn't be there.