The 57-year-old man, who was a Jetstar captain at the time, has pleaded not guilty to two counts of murder and is on trial at the Supreme Court in Melbourne.

Robin Ashlin, a friend of Mr Hill's for more than 25 years, told the jury on Friday he first met Mrs Clay at a camp site at Timbarra in 2019. 

Mr Ashlin had gone with his wife to visit another friend, James Francis, and Mr Hill was also there.

He introduced Mrs Clay as his friend but "blind Freddy could see the way they looked at each other", Mr Ashlin said.

Mr Ashlin never mentioned the meeting to Mr Hill's wife, but she later told him Mrs Clay had "been on the scene" for more than 20 years.

The pair had been childhood sweethearts and Mr Ashlin understood there was a pact between Mr Hill and Mrs Clay to divorce their respective partners so they could be together. 

"Carol divorced her husband, but Russell backed out," Mr Ashlin told the jury. 

He said he was shocked to find out about the affair because it seemed out of character for Mr Hill. 

"What is the go with one on the side - one is enough trouble," Mr Ashlin said. 

Mr Francis, who was friends with Mr Hill for a decade, said he met Mrs Clay several times as Mr Hill brought her on joint camping trips. 

It was his understanding that Mr Hill did not want others to know about the affair. 

Both Mr Francis and Mr Ashlin spoke to Mr Hill on March 20 - the day the couple were allegedly killed - through amateur radio.

Mr Hill was "like a pig in s***" and couldn't have been happier camping in the Wonnangatta Valley, Mr Ashlin said.

Mr Francis also told the jury Mr Hill seemed like his usual self in their last interaction.

The trial continues.