Would a married man get jealous if he saw his ex mistress wearing her engagement ring again?

Probably but he can't do anything about it.

ANSWER;

Would he, good question. Maybe he can because this married man love his ex mistress, but he doesn't have that right. He knew that he was married when he approach her before the affair, so what ever his ex mistress was doing, he doesn't have any say on that. He also ruin her life and no one put a gun in his head to have an affair with the other woman. It's all his, no one else.