I don't think so at all. Confidentiality has its place in grown up relationships.
Secrets are things involving YOU that you keep from your partner. Like an affair, or spending £2000 on shoes. Not information purely regarding other people that is not yours to share. The abortion is not her business and not her secret. It's the daugher's.
My husband has absolutely no reason to be told that some of my close friends have had abortions or abusive relationships. Or, say, my mum's medical history. I don't expect him to tell me everything his mum confides to him. We trust each other to be honest where it counts. I work in healthcare and can't tell him about my patients, fundamentally i see this as no different. Likewise a lot of jobs require confidentiality or NDAs.
The daugher isn't in their relationship and doesn't consent to her information being shared. If OP blabs, what lessons is she teaching about keeping promises, privacy, or consent? Or confidentiality? I'd argue that revealing it would be setting a poorer example. Putting aside that she's not a child any more and doesn't need patronising.
If OP told him, it could ruin her relationship with her daughter, potentially forever if she feels that her confidence has been betrayed. It won't be teaching the daugher anything apart from that she can't rely on her mum to keep her word and listen to her. And if the dad reacts unkindly, it could permanently affect her relationship with him too. People cut off their parents for far less.
It's hard for OP, but keeping quiet is by far the best choice here. The daugher may reveal it herself in good time.