Ok, after checking out your problem GTASAddict and OrionSR reply I did some testing to get dumped by Denise. After shooting her on the first date and hitting her on the second date I got her stats to 80%. She dumped cj on the third date, I saved the game.
phase 1: Used enable all girl friends. Denise heart icon returned but
stats in the pause menu do not return and she will keep dumping you
if you try to date her. So you are right about that.
phase 2: Used enable all girl friends + Denise stats to 100%. Her stats
returned in the pause menu so I got wasted. Went to date her and it
worked. So this 2 code combo will bring back denise
Notes: when I originally tested the enable all girl friends code a few years ago, I tested it on Katie. I kept killing her and the code kept bringing her back. I never thought it was a difference if cj is dumped or if a girl friend is killed.