The US presenter, who split with Cowell after six years in 2008, said that she even hopes to attend the wedding.
According to The Mirror, Seymour said: "I'm so happy for him - I'm delighted for them both. It's great to see Simon so happy. There's absolutely no awkwardness between me and him.
"He's like my best mate. I have to interview him all the time for Extra, covering American Idol."
Cowell ended weeks of speculation by confirming his engagement earlier this week and is thought to be planning an autumn ceremony.