Yes he should tell her. My relationship with my father was very similar to the situation you described between the daughter and father in your post. My father kept quiet. His new wife decided to let me know by sending a letter , addressed to herself, to me, at one of my first jobs, where I promptly fell apart.
While I'm sure you wouldn't do that, there may well be somebody who would find a way to let her know, or she could find out by accident.
When I discovered that he had effectively confirmed that I couldn't trust him to be honest, I ended the relationship immediately and never saw my father again.
re this "(due to XW doing her best to turn her against him)"
May well be true. I know my own mother did her damnedest to lay all blame, for everything, plus many other fabrications about what happened next, entirely at my father's door. So he had every right to claim that it wasn't as black and white as it had been presented to me. However it became a refuge for him to avoid taking any responsibility at all for the part he played in the break up and many of his actions afterwords. "Your mother is poisoning your mind against me" became a mantra and a shield against taking responsibility for what he DID do wrong. His inability to face up to my justifiable anger and disappointment was a huge barrier. Even if his new wife hadn't forced the issue I still think any relationship was at risk of imploding on that basis alone.
For all his former wife's bias, his daughter isn't entirely off the mark in being wary of trusting him and finding it hard to get past her anger. He is proving by his current choices that he doesn't naturally select openness and honesty with the people he has a significant relationship with. Something she is demonstrating that she is aware of by the rocky state of their relationship.
He is the adult, she is the child. It is up to him to take risks that leave him open to having to deal with the reactions to his having moved on while she is still stuck in the aftermath of her family having fallen apart. But instead he has chosen to exacerbate her pain (cos she will find out eventually) setting her up for a huge sense of betrayal when it is revealed that he lied by omission. If that happens she is the biggest loser, because at that point she is more likely to believe every less than truthful thing her mother has to offer, compounding her anger, hurt and loss. Making her even more susceptible to emotional manipulation by another parent that can't prioritize the child.
Ask him if he'd die for her, and if he says yes, ask him how that can be true if he can't even take an emotional hit for her sake, in the name of causing her less harm and pain in the longer run.
As she grows and sees things through a more adult lens she will become ever more capable of seeing things from his perspective and forgive transgressions on the basis that all humans are flawed and you can't ask for perfect behavior, in every situation, particular high tension ones, all the time. Honesty gives a better chance on that coming to pass with their relationship at a point where rebuilding can take place. Dishonesty could leave both of them open to comprehension coming about when she no longer feels there is much point to salvaging the tattered wreck of their non-relationship.