I love the idea that she thinks he's a part of this illegal activity. She doesn't tell anyone because she's conflicted about her feelings for him. She's actually in denial because the truth is she still loves him. I think him being undercover is a great idea. She would obviously not know this. I think you could also add the element of a police cover-up/dirty cops as Joshua suggested. Maybe they work together to try and uncover the truth about the dirty cops. I would save the "big reveal" for the end. Maybe at the end she finds out he is an undercover cop and a good guy after all.

Be blessed and be a blessing,
Fancy