Interesting thread,
I would say that if you have directed a player to the Casino and that is the players only account with them, and they then cross sell it to another casino in their Group you should be entitled to a revenue share.
If the player already has two accounts withinthe Casino group and has been directed their by 2 different affiliates it becomes trickier if they then cross sell it to a third Casino. On the other hand you would want to know why they tried to cross sell him to a new Casino instead of giving a great offer on the current one.
Also as Casinobonusguy says, it is hard to find out if a site cross promote unless you know them, or Why just that player is being targeted, he might have signed up for a newsletter or shown interest in the Casino somewhere else and therefor getting the Casino software and letter sent home.
Cheers
Henrik