Sidwell Friends School is being sued for $10 million for allegedly failing to supervise a staff psychologist who was having an affair with the married mother of a 5-year-old student he was counseling.

The woman’s husband and girl’s father, Arthur G. “Terry” Newmyer, made the allegations in a lawsuit filed Thursday in D.C. Superior Court, in which he also said that he had alerted the school to the alleged affair over the course of a year. In his lawsuit, Newmyer said his daughter suffered emotional distress as the affair became known among teachers and other parents.

Ellis Turner, a spokesman for Sidwell, said that “we are very disappointed that Mr. Newmyer chose this course of action” and said the school would “vigorously defend” against allegations that are “completely without merit.”

The counseling sessions with the child occurred off school property, the lawsuit states. But Newmyer’s wife, Tara Newmyer, said through an attorney that the school psychologist, identified in the lawsuit as James F. “Jack” Huntington, was not treating her daughter “in any professional capacity.”

In a letter to parents sent late Thursday, the school said it “does not believe that anyone it employed ever had a therapeutic relationship” with the Newmyers’ daughter.

Arthur Newmyer is an entrepreneur whose family has had a long association with Sidwell, where an annual award to honor volunteerism carries his family’s name. In his suit, however, Newmyer said that he had objected to enrolling his daughter at Sidwell and told his wife that she would have to cover the tuition. The girl remains in kindergarten at the school, the lawsuit states.

Huntington left Sidwell in February after a decade there, according to the court filing. The lawsuit contends that Huntington was fired by the school’s board after Arthur Newmyer shared sexually explicit e-mails that Huntington allegedly sent to Newmyer’s wife, some of which were sent over the school’s computer system.

Tara Newmyer’s attorney, Kristin Henrikson, said her client and Huntington became romantically involved while the Newmyers were separated.

Her client’s reaction to the suit was “shock and disappointment that Terry would choose to do this to their children and take what is a private family matter and turn it into a very public airing,” Henrikson said.

Turner said he could not discuss Huntington’s tenure at the school or the terms of his departure because they involved personnel matters.

Huntington said Thursday that he had “no comment” about the lawsuit’s claims.