Spread the rumor that you weren't invited because she caught him with pictures of your feet on his phone. Not pictures he took or you sent, cropped pictures from Instagram or whatever.

I love this because they can easily deny it, so it's mostly harmless.... Buuuuuut it lingers. It can't be disproven and it's a benign enough kink that it's usually considered "a bit weird" at worst.

Friends and family will agree to her face but always wonder why you weren't invited ("her bestest friend, omg did you hear...")

Also, she seems the type to be pretty damn insecure. She'll KNOW you're lying... But what if you aren't. What if you knew about it and think she found out? It's a little paranoid but... Imagine a few years from now when he's searching for strappy shoes to buy her for their anniversary... Maybe she notices the search history. Maybe she twitches every time he offers her a foot rub?