If what they're claiming is true... then it would seem that we have culpability on the part of Roobet for continuing to operate a "broken" system... but also culpability on the part of Yahoo for knowingly using said "broken" system. 
Scam? no... total clusterfuck? yes
Scam? no... total clusterfuck? yes
IMO what's most fair:
A settlement payment of some sort would be best because there is negligence in both parties. Paying out the full 9k is excessive but somewhere along the lines of x0.3 - x0.5 of the win amount would be appropriate.
Well if they specifically mentioned there was a bug for that specific problem and the onus was on the player to decide whether knowing that they wish to proceed, and whether they can not mess up and accidentally over bid. Then I see 0.3 as excessive.
But perhaps it " could " be worth it to them to offer 1k extra if he removes all the screaming about scamming and all other shit that does not make it clear HE made the mistake and he was aware such mistakes were possible on the players part if they chose to proceed during the phase the hole is getting plugged.
Really though they (casino) could have a valid case for calling extortion or blackmail on yahoo using the trust feedback here to leverage his "compensation" when it was his own decision to proceed and their own mistake, and making undeniably false claims of scamming him.
If yahoo got another 1k he should feel pretty lucky, and they should plug the hole asap.