I [26m] know an unfortunate truth about my girlfriend’s [24f] biological father [64m]. I don’t know what to do with the info.

I don’t really know where to start with this. It’s been eating at me for a while, though.
I’ve been with my girlfriend for three years. She is an incredibly warm, loving girl. She makes me feel more loved and cared for than anyone I know. We’re at the point now of talking about getting engaged, which kind of makes this all the harder.
My girlfriend is also kind of emotionally fragile. She has abandonment issues, largely stemming from her dad, who took off on her and her mom when she was very young. He sent child support, and that was all they ever got from him. It’s had its effect on her. She tends to cling pretty hard to me, her friends, and her mom.
She doesn’t talk alot about her dad. When she does talk about him, she talks in kind of an idealized way, like she assumes he really did love her but couldn’t come around for one reason or another. I assume its a coping mechanism.
Almost a year ago we were at her mom’s and she was showing me some random photos from her childhood. Some of them had her dad in them. It occurred to me that he looked alot like an old friend of my family.. I wasn’t quite sure enough to say anything, so I didn’t.
A couple months after that my uncle was getting a big time work promotion, and had something of a party at his place to celebrate. The gf wasn’t able to go for one reason or another, and I was just planning on going for a bit to show my face and then dip out.
While I was there I saw that old family friend and remembered the resemblence. I privately asked dad if [friend] had ever been married. Dad said he thought so.
I wound up talking to [friend], and basically asked him the same thing, if he had been previously married. He said he had, and that he had a daughter.
I told him I that I thought I know the daughter, he got a little dismayed. He basically told me that he felt a bit bad about it, but he had basically accidentally knocked up her mom, he had wanted nothing to do with kids. He tried to get by for a few years but ultimately couldnt do it, so he left. He paid the child support but never wanted anything more to do with his ex or their daughter, and he was still satisfied with that choice.
I never did tell my girlfriend about that. She still ocassionally talks about trying to get in contact with him, and maybe trying to have a relationship. How the hell do I tell her that her dad still wants nothing to do with her?
But then, I also believe in open and honesty with my partner. I don’t want to lie to her by omission. I know exactly why her dad left, and that his only regret was having her in the first place.
She probably deserves to know, but I also feel like maybe this is one of those truths that is best left alone. Knowing would crush her. Not telling her feels like a lie every time I listen to her talk about reaching out to him without saying anything.
Should I tell her? Should I just let it lie?
TL;DR: I know my girlfriend’s long-gone father wants nothing to do with her. She’d be crushed to know. Should I tell her?


Source.