The 19-year-old apprentice bricklayer from Lancashire in north-west England was last heard from on June 17.
He called his travel companion Lucy Law to say he was lost and attempting to walk back to their accommodation, but needed water.
"He wasn't aware of his surroundings, he desperately needed a drink and his phone was on 1 per cent," she said, on an online fundraiser.
"He had met two people on Sunday night and left with them to go to their apartment. Their apartment was miles away from civilisation and in a very secluded location."
Law said Slater was wearing only shorts and a T-shirt, inappropriate for both the warm daytime weather and cold nightly lows.
The case has attracted intense public interest in England, Spain and overseas as authorities combed the Canary Islands off the coast of Africa with helicopters, search dogs and drones.
Activity on his Instagram account briefly raised hopes last week before supporters said someone had "hacked" into it. They've also condemned the "trolling" of some people helping with the search.
Police announced a renewed "mass search" for the missing teen on Friday but on Sunday confirmed that had wrapped up.
"The search operation is no longer active, but the case is still open and there are several lines of investigation," a Civil Guard spokeswoman said.
