You know what? I'd be conflicted too if was in your shoes.
I guess that age doesn't matter where affairs of the heart are involved though.
If you really think that things are headed in the romantic direction I would suggest that you may need to decide ahead of time what the friendship is worth vs a relationship with this man.
She may be unable to maturely come to terms with this.
I would NOT bring it up until he actually asks you out (or you ask him out) at which time I WOULD bring up the friendship you have with his long ago ex.
And if he has no problem still wanting to go out with you, THAT is the time to bring it up with her if you are willing to risk that she can't handle being friends with someone she carries a torch for.
Sorry about all the long long sentences - hope they made sense.
If this relationship was a year or five in the past I'd probably tell you that a good friend is harder to find than a "maybe" relationship - but this was a long time ago!
Her feelings may still keep him more in the present - and that is where you would take the risk of losing her friendship now...