Don't let the fox guard the henhouse.
Don't assign the duty of protecting or controlling valuable information or resources to someone who is likely to exploit that opportunity. You're going to put your ex-convict brother-in-law in charge of your business? I can't tell you how to run your company, but don't let the fox guard the henhouse.
fox guarding the henhouse
A person likely to exploit the information or resources that he or she has been charged to protect or control. My sister is going to put her ex-convict brother-in-law in charge of her business, and I'm worried he'll be like a fox guarding the henhouse.
cock in the henhouse
The only male in an all-female environment. Getting a male teacher is a big deal when you go to an all-girls school! Finally, a cock in the henhouse! My grandpa always complains that he's a cock in the henhouse living here with my grandma, my mom, and my twin sister.
fox in the henhouse
Someone with bad intentions. (A fox would prey upon hens in a henhouse.) I'd watch out for him if I were you—he walks around here like a fox in the henhouse.