Kok Ksor, head of a Montagnard association in the US, died in the United States last week, according to Sam Rainsy.
Mr Rainsy tweeted on Saturday that he learnt of the death of Mr Ksor in the state of North Carolina in the US.
“I learnt of the death of my friend Kok Ksor with huge sadness. Kok Ksor was great leader of the Degar, an oppressed indigenous people from the highlands central Vietnam,” Mr Rainsy said on his twitter.
Montagnard is a French term referring to a Vietnamese hill tribe, also known as Degar, most of whom are from Christian ethnic minority groups residing in the Vietnam Central Highlands.
Mr Rainsy was summoned by Phnom Penh Municipal Court to appear at the court for questioning in May after he was accused of agreeing to cede four northeastern provinces to Vietnam’s Montagnards if he won the 2013 election.
The case stem from a leaked video on Facebook within a post that included documents dated April, 12, 2103 and bearing the signatures of Mr Rainsy and that of Mr Ksor.
In the video, Mr Rainsy said he welcomed Mr Ksor to visit Ratanakkiri, Mondulkiri, Stung Treng and Kratie provinces.