CNN is trying to prop up one of Bill Cosby's key accusers with unethical, malicious tactics ... so claims Cosby's lawyer.Marty Singer has written a scathing letter to CNN Prez Jeff Zucker, claiming his reporters are out to get Cosby in a special the network is doing with model/actress Beverly Johnson.Johnson claims Cosby drugged her in the mid '80s at his NYC home. Singer says in his letter -- obtained by TMZ -- he contacted the CNN reporters and told them they should get in touch with Mark Burk, who was Johnson's live-in boyfriend from 2006-2009.Singer says Burk told CNN reporters Beverly "never said a negative statement about Bill Cosby throughout their entire four year relationship." Singer goes on, "Burk told CNN that to the contrary, Ms. Johnson stated that she was an admirer of Bill Cosby and only said great things about Mr. Cosby."According to Singer, the reporters tried to pressure and manipulate Burk to corroborate Johnson's claims. He says when Burk stuck to his guns, the reporters abruptly terminated the conversation.Singer goes to town on Zucker, accusing the network of being "reckless and malicious." He says, "Accusers are being given a national platform by CNN without first exercising the most rudimentary journalistic investigation as to their claims and motivations."We reached out to CNN ... so far no word back.