The "girl's" done nothing wrong! She's free to flirt with who she wants. Blame your husband 100% for this for thinking with his willy, as most do.
I posted that ^ all the while knowing you've set your heart on doing what you said you were going to do. However it doing so, you have to take into account any consequences that it may bring. Your husband may see you for what you really are, a spiteful harridan of a woman, who's seeking to hurt a (presumebly) elderly couple who have done nohthing wrong to you.
I won't wish you luck, as its a horrible thing you're doing.