According to the Daily News, Matthew Lombardo says Harper and her husband concealed her lung cancer from him until it spread to her brain.
Harper had to pull out of last year's national tour of the play.
The lawsuit says the 74-year-old former "Rhoda" star knew in 2009 she had lung cancer but "knowingly withheld the truth." The suit was filed in Manhattan Federal Court.
It was a counterclaim to one Harper filed demanding she be paid for the production she left for health reasons.
The Daily News says Harper couldn't be reached for comment.