Bob Lindsay said his son Shawn Lindsay, 43, let Joe Sylvester go from a job at his oilfield company in Consort, Alta., two weeks ago.
Lindsay said his son's girlfriend told him that the couple awoke early Friday morning to find Sylvester in their bedroom with a rifle.
"Shawn woke up and Joe was standing in the bedroom door," Lindsay said. "Shawn got up and he threw him out of the house."
Lindsay said his son locked the door, but Sylvester smashed a window and got back inside.
Lindsay said that after the shooting he brought his son's two boys, aged 5 and 17 months, back to his own house. But then he recalled that the car sitting in front of his son's house belonged to Sylvester's girlfriend, Christine Clifford.
He tried to phone Clifford but got no answer. He then got word to the police that they needed to go to Sylvester's trailer right away to make sure Clifford was safe.
But Lindsay said officers found her dead.
RCMP have not confirmed the identities of those killed and have released few details. They've only said all three people were known to each other, the public is not at risk, and that investigators were not searching for any suspects.
Lindsay said his son wasn't killed immediately — he was shot in the stomach and died three hours later.
He said Clifford, whose husband died of a heart attack two years ago, had four daughters.
Lindsay said Sylvester had worked for his son's oilfield maintenance company for several years and that the two had been friends. But he said Sylvester's outlook had become negative and it was affecting his relations with co-workers and customers.
He said his son had told relatives he was considering firing Sylvester for months. The two had a fight last month and Lindsay says his son told Sylvester that he was through and not to come back.
Lindsay said his son told neighbours that he was afraid of Sylvester, saying that one day he was going to "flip out."
He said he doesn't know why his son didn't take Sylvester's rifle away before kicking him out of his house.