My husband's ex left the area, and returned right before we got married. DH went to see her then without asking me. I admit it. It bothered me. I'm something between happy about that now and still remembering it bothered me, because he didn't tell me until after we were married. And, yes, he did go see her to make sure there was nothing left between them. Not as bothered now, because there wasn't, but still think he should have told me.
She was a member of the same church we were members of for a couple of years, and I got to know here better. Nice lady. We became causal friends. (After all, I won his heart. I truly get that.) And then we left the church when we moved.
They met, yet again, through FB. We've friend each other.
I trust him. I trust him because of the decades of marriage we have had. He is trustworthy. For that matter, although she really regrets their relationship ending, she is trustworthy too.
Which makes me wonder why your husband doesn't trust you around an ex from 25 years ago. I can understand there being hesitation if you were planning a date with him, but you're talking about your father, someone you both deeply cared about. And something that can be done on the phone in front of your husband. Is it he doesn't trust you, or just that you fear he doesn't? If it's fear, talk to him about it. As it stands now, you're hiding this from him, which is not a trustworthy thing to do.