A Manhattan hairdresser told an Upper West Side mom not to wet her treated hair — the day before she was found dead in her bathtub.

In testimony reminiscent of a scene out of “Legally Blonde,” Adam Aminov told jurors Monday that he instructed Shele Danishevsky to avoid any dampness for three days.

“The most important part after finishing [the treatment] is not to get the hair wet or any humid [sic],” testified Aminov, who performed a keratin treatment on Danishefsky the day before her 9-year-old daughter found her lifeless body on New Year’s Eve 2009.

Prosecutors point to the testimony as further evidence that Danishevsky’s estranged husband, Rod Covlin, killed her and placed her in the tub.

“She know that. Everybody know that,” the Uzbek hairstylist said at the Manhattan Supreme Court trial.

In the 2001 film, a Harvard lawyer played by Reese Witherspoon, pokes holes in the testimony of a witness who claimed she didn’t hear a gunshot because she was washing her hair.

Inspired by a conversation with her hairdresser pal, Witherspoon’s character points out that the witness had a perm hours earlier and would know that shampooing her hair would deactivate the salon treatment.

On cross-examination Monday, defense lawyer Robert Gottlieb asked Aminov whether he explicitly told the UBS wealth manager that she couldn’t bathe for three days. “No,” he replied.

Later, real estate developer Neil Rothenberg said that he had gone on two dates with Danishefsky, 47, weeks before her alleged murder.

She told Rothenberg, who she met on JDate, that she feared her husband and was considering paying him $250,000 to “make him go away.”

On their second date, just two days before her body was found, he said Danishefsky was in a rush to get home by 8 p.m. to pick up her two children.

She was concerned that if she was late, Covlin, who was living across the hall from her, would use the key she had given him to access her apartment.

Rothenberg said that he thought it was bizarre Covlin had a key when Danishefsky had a protective order against him.

Assistant DA Matthew Bogdanos has argued that the backgammon-loving womanizer sneaked into Danishefsky’s apartment, strangled her, and then made the scene look like an accidental drowning to get his hands on her $5.4 million fortune.

On Jan. 6, 2010, Rothenberg learned of Danishefsky’s demise when he called her office after he hadn’t heard back from her about the third date they had planned.

“They said she’s not here,” recalled Rothenberg. “I asked, ‘Can she get back to me?’ and they said, “‘No, because she’s dead.’”