The KPNLF were in a coalition government from 1982 on with Sihanouk and the communists, whom you call the "Khmer Rouge". In fact the coalition with the communists happened as soon as the US began funding them - it was probably ordered by the US.
Any how, the US fairly openly armed what you call the Khmer Rouge, I guess a sliver of plausible deniability is enough for the minority of people like you who want to deny it. Nightline even did a whole segment on it.
Conflating the KPNLF with the Khmer Rouge, a group they were literally formed to fight - whose members suffered during Pol Pot's reign of terror, because they joined a coalition government in exile for expedience is just nonsense. The Red Khmer were Communists.
The KPNLF were essentially strong armed into a coalition government by the Thai - who threatened to cut them off from aid and boot them from the country - and to gain international recognition and aid. They waited until the last possible moment to join and remained hostile towards the Khmer Rouge.
If you have a link to this Nightline episode, I'd love to see it. I couldn't find it.
Any how, the US fairly openly armed what you call the Khmer Rouge, I guess a sliver of plausible deniability is enough for the minority of people like you who want to deny it. Nightline even did a whole segment on it.