Felicity Loveday was a follower of Buddhist mysticism and meditation who believed she had rid herself of "bad magic" on a trip to the Dead Sea.
Her family believed the bad magic had returned in 2019, but by then the 83-year-old with a number of health conditions including dementia was too unwell to rid herself of it again.
So her son, Adrian Meneveau, took it upon himself to do it for her. His sister Christina said the belief was the bad magic could only be put "to sleep" by being "out on the salt water".
Mr Meneveau bought a boat, cancelled his lease, gave his sister a copy of his will and set off with his mum on a journey that had a fatal outcome.
A Victorian corner has found there's no evidence Mr Meneveau intentionally ended his mother's life on that boat trip, but that he did contribute to the death of the "vulnerable and helpless woman" sometime after December 11, 2019.
The pair was last seen by Ms Meneveau on that day when she waved them off from a boat ramp on Port Phillip Bay in a small fishing vessel, better suited to lake conditions.
She took pictures of her mother and brother, wearing life vests. Ms Loveday was sitting in a front seat of the uncovered boat while her son was moving provisions in the back.
They were expecting to be gone for two or three days. Ms Meneveau reported them missing on December 14 and their capsized vessel and life jackets were found a short time later.
Coroner Audrey Jamieson found after an inquest earlier this month that both died on that trip, though she was unable to say how or when.
In findings handed down on Monday, she said Ms Loveday was a former secretary of Buddhist mystic Ferdi Rossi, whose beliefs she also followed.
A month before the boat trip, Mr Meneveau wrote to Mr Rossi's successor James Walter, telling him his mother was "now experiencing her childhood and is no longer in logic" and that he was working to "detach her pantry in Egypt".
Ms Jamieson said evidence suggested the latter remark referred to his intention to dispel the "bad magic" he believed had been incited by his mother.
She said there was insufficient evidence to support a finding that Ms Loveday's life was intentionally ended by her son, or that he had intentionally ended his own life.
"But equally the possibility cannot be sensibly excluded," she said, noting their life vests were found with the boat wreckage.
While she was also unable to find whether Ms Loveday removed her own life vest, she said the evidence indicated Ms Loveday may not have been able to do it alone.
"The weight of the available evidence indicates that Felicity Ruth Loveday was rendered vulnerable and helpless by her disabilities and her dementia," she said.
She found by taking his 83-year-old mother out on Port Phillip Bay in a vessel not suited for that purpose, Mr Meneveau had contributed to her death.
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Australian Associated Press