The 43-year-old British actress discovered an old postcard at her mom's house Wednesday that features Lowe's imagined response to her marriage proposal.
"EITHER I was a really tragic 13 year old with time on my hands OR Rob Lowe was all kinds of casual proposing marriage via a postcard of himself," she captioned a photo of the card. "(found at my mum's house)."
Lowe, now 53, skyrocketed to fame after appearing in such films as The Outsiders, Oxford Blues and St. Elmo's Fire. He married Sheryl Berkoff in 1991, and shares sons Matthew and John with the makeup artist.
"I was talking [with my sons] about some young actor the other day who's like the 'it guy,'" the star told Yahoo! in October. "And I said, 'You know, I get it, I've been there.'"
"I looked over at my kids and they were literally rolling their eyes and making the gag motion. I finally had to turn to them and [say], 'No, no, listen. Google it! It's the truth,'" he said.