The actual invasion ? Like from December 78 to January 79? Considering how quickly it happened that would be bizarre. No. The US government did not directly assist the Communist Party of Kampuchea. I guess it depends on your definition of ‘directly’ but there was certainly nothing amounting to US military assistance to the Khmer Rouge, and nothing in the period before or after the actual successful toppling of the DK regime.

US aid to a coalition that was made up significantly from the Khmer Rouge was more or less just financial and within the UN in the months and years following the Vietnamese invasion, not during. It was a depressing example of realpolitik.

Source

‘Getting away with genocide’ Fawthrop and Jarvis

edit

Haha, yeah I thought that might have been the intention, why not check out answers that were put to this question about US involvement with the Khmer Rouge