Does she have a celebrity crush or social media star she thinks is attractive? You could mention having a dream where she had sex with him. Even better if she has like a coworker or someone you guys know she thinks is cute.
From there you can play around with it to fit her needs. What was the dream?
My husband falls asleep before me. One time about two hours after he fell asleep, I heard him moaning. He woke up kissing on me and told me had a dream about sharing me with an ex of mine. It turned us both on and we had an impromptu role play session.
He later admitted to me there was no dream he just faked being asleep and waited on his time to strike lol.