A police investigation is under way, although an inquiry by the executor of her will found "no evidence which would enable me to found a claim against Brendan".
Mr Horan denies any wrongdoing and says Mr Henderson was feeding information to Mr Peters.
A letter written by Mr Henry, a friend of Mr Peters, says the defamation occurred during an interview with Mr Horan on RadioLive about the allegations levelled against the MP.
In his letter, Mr Henry said his client sought an apology and $300,000 in settlement.
The Herald understands the case has now been filed seeking "general damages" from MediaWorks and $350,000 in damages from Mr Horan.
Mr Henderson was convicted in 1997 on 30 sample charges of smuggling and 30 charges of wilfully making false import entries to avoid tax.
Mr Peters spoke at the NZ First conference at the weekend about a "clampdown on tax evasion" as a way of funding the party's policy of removing GST on food.
He confirmed that he had spoken to Mr Henderson, 66, about Mr Horan but wanted to make no comment on any matter before the courts.
Mr Peters acknowledged that Mr Henderson had passed on information about Mr Horan but when told that his tipster was an undischarged bankrupt with tax-related convictions, said he had never acted on any of it.
In Parliament yesterday, Mr Horan said any money Mr Henderson had to spend on lawyers should be paid to his creditors.