"I don't usually comment on my friends' personal lives," Prepon said when asked about the duo. "But they're an amazing couple."
Prepon's declarations came a day after it was reported that Kutcher spoke to Kunis' dad before asking her to marry him.
"He asked for Mark's blessing to marry her. It was important to Ashton to be respectful and traditional. This is the real thing for him and he wants to do it all right," a source close to the Two and Half Men star told People.
While the exact date of the engagement is still unclear, Kunis began being photographed with a diamond ring on her finger in late February.
2014: The Year in Fashion [PHOTOS]