Well, Nelson went back to Jonathan's apartment that night, so she either left him (alive/deceased)at her house when she did so or she had him (deceased) in the truck and put him in the ditch.
She must have went back to the apartment for a purpose that night.
Besides placing herself in the apartment in case LE found evidence of her there.
Speaking of which, DID LE find evidence of Nelson in Jonathan's apartment?
I would not be surprised, when and if it all comes out, that she was there as an 'Enforcer' to let mom know that it was pay up or else. And sadly, it turned out to be 'or else'.