Chloe almost suffered from a heart attack after seeing that Lucifer does indeed have a twin brother. The elder twin, Michael who assumed the former consultant's identity in an attempt to take the life Lucifer built for himself.
The plan was thwarted nonetheless, from Amenadiel's effort to go down and get the Devil, to the collective effort — rather, doubts — of his closest friends, Linda, Ella, Maze.
Most especially, Chloe. Chloe who at first believed all the lies but later on saw through the facade and called the impostor out.
Lucifer did the rest.
She thought that was the end of it. That he only came back to fix the mess his brother made. When he invited her over to the beach, blindfolded, she thought he was going to say goodbye again. Her heart couldn't handle that so she decided to end it for both of them.
The Detective took her necklace off, the one he gave for her birthday, something that she cherished ever since. She didn't want to part with it, however, if she gave it back to Lucifer, then he would have something to remember her by.
So she did. It broke her heart to see him so heartbroken and confused upon the receipt of the valuable item. Chloe meant to explain, yet her tears threatened to fall so instead, she turned her back and tried to walk it off, in hopes that he'd follow. And Lucifer did. He wound her in a tight embrace and whispered the words she had been meaning to hear him say. Though it had a finality to it and before she could process what was happening, he was gone.