While reading the book I accepted without question Ralph's death bed statement that the repairman raped Julia. Thinking about his and his girlfriend's unattractive appearance and his mundane job we can assume that the repairman suffered from low self esteem making him suseptible to advances, whether real or imagined, from an attractive bikini-clad pubescent girl. And with a little alcohol in him, Julia's familiarity from the beach house and the party atmosphere on the beach on a summer night the ingredients were in place for a tryst in the sand that went terribly wrong. Moreover, Marc observed scratches on the repairman's body. Bottom line - I found Ralph's assertion to be very believable.
But the preceding responses have been most illuminating to me. I now wonder whether Ralph, knowing that Marc would be responsible for his death, lied to Marc as his ultimate act of retribution. I still believe that it is more likely than not that the repairman, as opposed to Ralph himself, Stanley or Alex, was the culprit. But I'm not sure.
Just another dimension to a very good book.