I'm not entirely sure who his best friend is, as he likes to keep his personal life quite private for the sake of his family (and friends), but I do know that at one point he was quite good Friends with Winona Ryder (there was a rumour that they were dating, but it was never falsified ou proven true), and he dated Anna Friel whilst making A Midsummer Night's Dream with her.
Currently, I'm not sure if he's still Friends with Ryder, what with that whole theft debacle and the fact that he met Blazic through her.
But yes, to the extent of my knowledge, the public is not privy to that information, and that's how he'd like it.
