At which point did Walder know that Roose was involved in treachery?

[Prince Yourwetdream Aeryn]

If Walder told Robb about treachery would Robb believe him or Roose? Even if he believed him cuting down all Bolton soldiers would weaken his power. If he destroyed northern Boltons with word of Frey he would lose his prestige among northern lords.

[Roswell]

Roose would not reveal something like that to Walder Frey until after Robb betrayed the Freys.  Roose found an ally after the Freys left the battlefield when they heard of Robb's marriage to Jeyne.

[Dorian Martell's son]

1 hour ago, Dead headofMaelysKinslayer said:

If Walder told Robb about treachery would Robb believe him or Roose? Even if he believed him cuting down all Bolton soldiers would weaken his power. If he destroyed northern Boltons with word of Frey he would lose his prestige among northern lords.

 

1 hour ago, The Transporter said:

Roose would not reveal something like that to Walder Frey until after Robb betrayed the Freys.  Roose found an ally after the Freys left the battlefield when they heard of Robb's marriage to Jeyne.

This Roose did not seem to go to battle with the intention of screwing Robb, but when the opportunity presented itself, he took it.