Barry Steenkamp revealed his regret over failing to drive Oscar Pistorius out of his daughter Reeva's life, before she was killed by the runner.
Barry, a horse trainer, admitted doing so would have made him unpopular with Reeva - but he would not care about it.
Speaking to Hello magazine, Barry said: "If I'd known what I know now, I would have attempted to stop the relationship.
"Reeva wouldn't have liked me for attempting to interfere, but as it turned out she would have thanked me in the end."
Barry has stayed away from the murder trial in Pretoria because of fears the stress of witnessing proceedings could be bad for his health.
He suffered a stroke in the wake of Reeva's death and is in fragile health.
Elsewhere in the same interview, his wife June Steenkamp revealed she has "forgiven" Pistorius for gunning down her daughter, having previously described the failed star as a "devil."
She said: "I don't hate Oscar. I've forgiven him. I have to - that's my religion."