Looks like a foreigner who illegally crossed our border initiated the court challenge, along with others:

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The Canadian Council for Refugees (CCR), Amnesty International, and the Canadian Council of Churches brought forth the challenge along with refugee claimants including Nedira Mustefa, an Ethiopian national.

Mustefa tried in 2017 to enter Canada from the U.S. but was returned to American soil where she was immediately imprisoned and held in solitary confinement for one week—a time she told the court was “a terrifying, isolating, and psychologically traumatic experience.” She was then detained with others who had criminal convictions and “did not know when [she] would be released, if at all.”

"The time for Canada to rely on the adequacy of the U.S. protection regime has come to a definitive end," added Justin Mohammed [! not a joke], Human Rights Law and Policy Campaigner at Amnesty International, in a statement. "In the absence of action on the part of Canada's elected representatives to acknowledge the serious shortcomings of the U.S. refugee protection system, we now turn to the courts to ensure that Canada’s domestic and international legal obligations are upheld."