He never told them.
Everything was bottled up-bottled like the alcohol he frequently sought solace in; at first occasionally, just to sleep at night, but then to keep the nightmares during the day at bay. It was engrained in his brain, don't show feeling. His survival in the Quarter Quell had depended on that- instead he had played the roll of the dark horse, managing to keep his emotions in check and outsmart everyone in the end.
Basic rule of survival. No feelings. Even though the Games were over, they weren't. Every day was just another part of the Games. The Capitol's games. Every day, he had to fight for his survival and sanity. Emotions constantly in check. Alcohol helped. A lot.
So he never really had the chance to tell them.
He had watched in anticipation with the rest of Panem, as the star-crossed lovers battled it out in the arena. He felt a twinge of guilt for not giving them better advice. He probably would have felt more, but he had been sneaking nips from his flask. Not too much. That was the best he could do for them at such a late time. Stay sober enough to help them with gifts. They had collected enough sponsors, all right, but he balanced out the gifts, praying the girl was clever enough to figure out how to earn them and get more sponsors. Stay alive.
She was.
The moment they had pulled the berry stunt was when he knew, they were just as clever as he was. He had managed to keep her alive, and she-well, she was outsmarting the Games and the Capitol. She was the one they had been waiting for. The one to lead them out of that nightmare.
He watched them emerge from the arena, victorious, and knew that it was all right. He had been able to help them, but in the end, they had listened to him and helped themselves. And now, everything would be different. Now, the Districts knew that the time for change had come. The nightmare might finally end. All because of those two kids.
And Haymitch was proud of them.
But he could never tell them.