After a day working on restoring his sports car, he had left their Queenstown home on Sunday night to get dinner but never returned.
Miss Horrell said she knew from the phone call, Mr Simon (37), a diabetic, was in trouble.
"It was on the way home it went wrong. I knew by the phone call I had with him he was unwell.
"I tried to ask him where he was. He was confused and couldn't tell where."
Mr Simon said the word "Goldfields" and Miss Horrell expected him to return 10 minutes later.
But after 20 minutes, she drove to Goldfield Heights, in Queenstown, expecting to find him stopped at the side of the road.
But she found no sign of him and an hour later, after calling friends to check his whereabouts, she reported him missing to the police.
Police, firefighters, coastguard, friends and colleagues conducted an extensive search until early the next day, but no trace of Mr Simon was found.
Later that morning, searching helicopter crew spotted the partially submerged vehicle in Lake Wakatipu and his body was retrieved that afternoon.
Miss Horrell, who works in marketing, and Mr Simon moved in together three months ago after meeting at his New Year's Eve house party in 2008.
He was determined not to let diabetes beat him.
"He was a fun, cheeky character, larrikin, one of the boys," Miss Horrell said.
"It's always been a very easy and comfortable relationship and there was so much more we were supposed to do."
Miss Horrell was being supported last night by friends and her parents, Elizabeth and Bruce Horrell, of Lake Hawea.
Mr Simon's father, John, and stepmother, Jan, of Dunedin, with brother Nathan Simon (32) and his fiancee, Shelley McGrath, both of Christchurch, gathered in Queenstown and travelled to Dunedin last night, ahead of the funeral on Friday.
Glenn Simon's mother, Jan McBain, who lives in Townsville, is also returning to Dunedin.
John Simon said he felt "lost".
"We're accepting and thinking of the good things."
Mrs Simon said she was feeling "hurt, angry, all of those kinds of things".
"He was a lovely boy who will live in our hearts forever."
Queenstown police confirmed yesterday Mr Simon was found inside a 2010 Ford Fiesta, which had left Glenorchy-Queenstown Rd, 33km north of the resort centre, opposite Pig Island.
It was believed Mr Simon was "suffering from complications relating to his diagnosed condition of being a type 1 diabetic", Acting Sergeant Blair Duffy, of Queenstown, said in a statement yesterday.
A Heliworks helicopter lifted the vehicle on to a flat-bed truck yesterday morning.
It will be examined by a vehicle inspection officer.
Mr Simon, who was born in Dunedin and educated at King's High School, was a qualified printer and had lived in Queenstown for seven years.
He had been named a printing apprentice of the year, and he worked at several Dunedin printers, including Allied Press Ltd.
He was affectionately known as "Gadget" because of his gift for mechanics.
He was diagnosed with type 1 diabetes during his overseas experience in England, the family said.
Mr Simon worked as a production manager at Print Central for five years.
He became an account manager, after Christmas, to experience a different side to the industry and worked with clients from Queenstown to Stewart Island.
Print Central managing director Graeme Hastie and colleagues joined the search for Mr Simon on Sunday night.
"We're in total shock, of course. Whilst we were aware he was diabetic, we would never anticipate something like this ... It was not an issue in performing his job."
Mr Hastie praised Mr Simon for taking "a huge amount of pride" in his work.
He had "old-fashioned values" and was intensely loyal to colleagues, friends and family.
"He was well respected at Print Central for his abilities and he's left a big void with his experience in the industry."
