Barba should be free to play as its an NRL ban only.
If he had stayed in RU for longer lets say a season or two with Toulon then joined Saints would he be banned from RL then? I very much doubt it. Part of the trouble I think is that the ban was only issued in the Autumn last year and the RFL are all too easy to fall into line with the NRL.
Ironically, the RFL's own sanction for cocaine use outside of the season is in fact much softer and less tough then the NRL. It is actually an internal disciplinary matter for the club. http://media.therfl.co.uk/docs/Socia...inal%20PDF.pdf