That's a tough one to call.

As the garage had undertaken the work requested, they are IMHO not liable for failure of another item that might cause problems with another fitted.. They may have checked the oil cooler which may have been working fine at the time?

Although on the other hand, if they know that a dodgy oil cooler could affect the new recon box, then they should have advised you of this, even just to clear blame if there was a problem like this.

It sounds a classic 6 of one and half a dozen of the other to be honest.

I hope you can reach a satisfactory outcome with this