Hi Legolas, and welcome to the site. It's great that you are hear looking for ways to help your friend, as I'm sure he could use your support in this. There are several great resources in the way of books - the first I would recommend is Mike Lew's "Victims No Longer" and the second is Mic Hunter's "Abused Boys". Both are arguably the best texts out there on the subject of childhood sexual abuse of males. And I would also recommend that you read these, as well as your friend. There are sections regarding friends and families of the abused boy that give powerful insights into how all of this spills over to them as well.
Again, welcome to the site, and thanks for being there for your friend.