Frederick Bello is the grandfather and rapist of Leslie Bello, infecting her with gonorrhea as a result of the rapes. Frederick was arrested and taken to trial, with Stabler presiding as a witness due to being assigned the case. Even Leslie testified to the rapes she suffered.
Casting reasonable doubt with other suspects in the jury, Frederick's lawyer swayed the jury to acquit. This shook Stabler and made it harder to work on cases at the time. (SVU: "Redemption")