Jonathan DeLaura, 35, is in custody, accused of working under an assumed name.
Nora Zetik says she first grew concerned when her 12-year old daughter complained of profanity-filled outbursts during lessons.
In 2009, DeLaura was convicted of having oral sex with a 12-year old boy while working as a tennis club instructor in Riverdale. He was sentenced to six years probation.
Specific conditions barred him from working at health clubs or with children, but early this year he began coaching at the Putnam Fitness and Raquet Club under an alias, even printing business cards with the name Jon Dulak.
Westchester County probation officials say that's an instant red flag.
Westchester's sex offender unit arrested DeLaura after getting a tip.
Club management had no comment other than to say it was aware of both names and ran background checks on both.
DeLaura is a level 1 sex offender, the least serious. Names of level 1 offenders often do not show up on public registries.
DeLaura is being now being held without bail.
There are no known accusations against him at the club in Carmel.