Okay it lands in a melted state and Rey finds it on her scavenging travels,not realising what it is, repairs it and makes it more powerful and then becomes intrigued with the force and the backstories and realises she has a natural ability with the saber, yet to realise that she is indeed force sensitive.
Or she just finds it one day partially buried in the dirt. We don’t know whose it was. Some Jedi, maybe, who died in the Clone Wars, or something. Its origin is unknown. She finds it one day, and it changes her life.
Cue the ‘helper’ whilst selling the crap she collects another strange hooded seller notices the saber on her belt as does the bar seller who asks to buy it, she refuses, he contacts his associates (as in TFA with BB8) to get the saber. A she walks away we see an attempt to take the saber from Rey thwarted by the mystery character who produces a light saber and strikes them down. First she feels threatened, until the mystery character assures her she is safe.