My Mom's New Boyfriend (2008)

His mom is dating his suspect. Let the surveillance begin.

After several years away, FBI agent Henry Durand returns to find his mother, Marty, a much different person: Having long mourned for Henry's father, she is now embracing single life. One day, while Marty, Henry and his new fiancée, Emily, are out for a walk, they meet Tommy, a mysterious stranger with whom Marty then becomes involved. However, through his FBI contacts, Henry learns that Tommy is suspected of being an art thief. As a young federal agent, Henry is given a difficult assignment: spy on his mother and her boyfriend who is suspected of leading a gang of art thieves.