Luke trained with a non-Jedi who was just an average person: Luke is further startled when she's revealed not to be a Jedi; she views the Order as trouble, and she educates him on the Force as something everyone can harness. As they hide out in the planet's caves, she warns him the Force is a dangerous tool, which basically alludes to the corruption of the Dark Side. However, Luke can't believe an "average" person is doing what Warba does