Trying to avoid posting too much on this issue as I'm so biased. However, trying to take an objective view.
1. As long as Sale haven't slipped up, it will be difficult to prove they have induced him to breach his contract.
2. The player has breached his contract. This is dealt with at an Employment Tribunal. Castleford should make a claim against the player as an individual and seek to recover their financial loss. If an ET find the player to be in breach of contract, they should be able to make a judgement on the financial loss suffered by Castleford using information such as previous transfer fees paid, the value of the offers Castleford turned down for the player and the loss of direct player sponsorship. ETs make far more complicated financial decisions than is required in this case.
3. Taking joint action against the player and Sale Sharks may be counter-productive. In this case, the player and Sale would be joint respondents, allowing the full financial weight of Sale to be used to defend the case. If Castleford only name the player as the respondent, any financial assistance given by Sale to defend a personal claim against him should be counted in their salary cap. It also means Castleford are likely to come up against much stiffer opposition in the form of Sale's legal team. Making the claim against the player only encourages puts more pressure on Sale and the player to come to an agreement with Castleford.
4. No ET in the land will accept this 'retirement' response. Given his age, his earning potential and his his immediate re-employment he has not retired.
5. Regardless of the above, his retirement is still a breach of contract. To retire, notice period still has to be given (unless this is due to illness). He hasn't done this, so legally it's still a breach of contract.
In summary, I don't think there is anything Castleford can do to prevent the player from gaining new employment. All that can be done through ET is claiming back the financial loss Castleford have suffered due to the player's breach of contract.
There is the option of lodging a dispute with the Court of Arbitration for Sport, but I have no idea how that works and what their powers are.
I also wonder if the player is being used by his agent and Sale as a test-case, and the player isn't intelligent enough to realise it.
If they get away with it, it lines his agent's pockets and gets Sale a free player.
If they don't, it's the player who is likely to be sued for breach of contract.
It seems like a no-lose situation for his agent and Sale, but a very risky one for the player.
One thing is for sure: all RU clubs will be interested in the outcome. If this is allowed to happen, it's an open market on RL players.