I want to make sure my friend isn't putting all his eggs in a broken basket, or at least that he is aware of the baskets flaws.
If that is the goal? Could deal with him direct.
You could make your friend aware of what you heard. And then leave it to him to decide what (if anything) he feels like doing about it. And have faith in the friendship -- that it isn't flimsy and a simple honest question isn't going to be a big thing.
Could keep it simple and honest. Maybe something like...
"Look, you and I? We're exes. That's not changing. We're friends.
As your friend? I wanted to make you aware that I heard Erika is seeing Ned. You seem to believe that you and Erika are not seeing anyone else. I'm not trying to make waves -- I want things for you and Erika to go well. What I heard could be wrong and behind the times. I hope so.
I didn't know what to do and you never told me your preference so I'm going out on a limb here. I'm just giving you the heads up so you are aware.
Could you reassure me that this is old news? Could you tell me what you prefer I do as your friend when I hear things in future? Should I let you know or ignore it or what? I need guidance. I hope I did not step on toes."
And leave it to him to decide what to do about it and clarify how he wants to you behave in future when you hear things that might concern him.