Could anyone please confirm to me the official ruling if a player is taken off injured. In particular when playing trebles, and one of the opposition injured her throwing arm to the point of not being able to continue.

We allowed the opposition a replacement player from the nominated team players for the subsequest ends. Was this correct?

I think you can do one of two things:

A) make the players of the opposing team use their "non" dominant throwing hand.

or

B) Break the wrist of a comparable player on the opposing team (to even things up).

The only reference in the rules is:
Article 34 – Replacement of a Player
The replacement of a player in doubles, or of one or two players in triples, is only permitted up to the official announcement of the commencement of the competition (whether vocally or by whistle, gun, etc.), and provided that the replacement/s was/were not previously registered with another team in the same competition.

However, some events allow replacement of players. It all depends on the organiser's rules for the event, e.g. our local "regional qualifiers" allow teams of 4 and a player may be changed during a game.

Regards,

Ray