James Learning, 74, was arrested Friday with seven other protesters who slowed traffic on a highway near the $7.7-billion development.
Roberta Frampton Benefiel, Learning's companion since 1992, said he is refusing food to fight a court injunction that she says restricts access to traditional hunting and trapping lands around Muskrat Falls.
The retired long-haul truck driver stands about five-foot-seven and weighs less than 63 kilograms or 140 pounds, Benefiel said from Happy Valley-Goose Bay, N.L.
She said he is also battling prostate and bone cancer, and has told his three grown children that he is prepared to die trying to defend his aboriginal rights.
"He went to visit his children in February and he said good-bye to them," Benefiel said through tears Tuesday as she waited to see Learning at the Labrador Correctional Centre. He is still in jail pending a court appearance Friday after refusing to sign a release form to respect the injunction.
"He explained that he was willing to go as far as necessary, and they said they understood and they support him," Benefiel said.
"I can't tell you if his mind would change at some point. But right now, it doesn't look like it's going to happen."
Learning is a member of the NunatuKavut Community Council, representing the Inuit-Metis of southern Labrador.
The group's president, former Liberal MP Todd Russell, was also arrested Friday. He is warning that protests will escalate as the province refuses to negotiate with him.
"This is serious stuff," he said Tuesday in an interview. "There is a conflict here, obviously, between this development and our aboriginal rights.
"If the government does not own up to its duty and the honour of the Crown to sit at the negotiating table with us, which is what Mr. Learning in part is fighting for — some fairness, recognition, some equality for God's sake, in this country — then they can expect more of it."
The province says it has consulted with Russell but has refused to offer Muskrat Falls benefits while his group has no land claim recognized by Ottawa.
Learning was the only protester still being held at the Labrador Correctional Centre after refusing to sign the release form, Russell said.
A provincial Justice Department official says inmates can't be forced to eat, but medical care is provided to monitor fluid intake.
"The well-being of the inmate would be of utmost priority," Luke Joyce said in an email.
The RCMP arrested the eight people during an early morning protest Friday that backed up traffic on the Trans-Labrador Highway leading to the Muskrat Falls construction site.
About 25 participants were handing out pamphlets stating that the hydro project was approved without proper consultations with the Inuit-Metis.
NunatuKavut, formerly the Labrador Metis Nation, has protested on the road before.
The Mounties said Friday's demonstration was peaceful but protesters who wouldn't co-operate with police were arrested and charged with obstructing a peace officer.
Electricity from Muskrat Falls would be sent to Newfoundland and Nova Scotia using vast transmission lines and subsea cables.