It sounds as though he was determined to go and try his hand in the NRL, so there's little you could do to stop his departure. What intrigues me is how you'll replace him or, probably won't. With the ever increasing NRL salary cap, plus the strong dollar to pound exchange rate tempting the very best NRL players to SL is getting increasingly difficult. Particularly when they'll also have to give up their representative career and SOO football to come over here.
As for Graham it will make him a better player IMO. Playing in competitive games week in week out won't do anything other than. I know Sam Tomkins only just signed a new 5 year deal last year, but come the end of that when he will be truly in his prime. I'd fully support him if it was his wish to play in the NRL. By then he'll have given Wigan many good years of service, just like Graham has for you guy's.