Justin and Zach
Well, when you put it that way, you're probably correct. It's good that this happened later on - I don't see Hannah as one to take tom-catting around lightly.
Just treat her right, and you've got nothing to worry about. Han's got a huge heart, we all know, and you're very lucky to have her. Someone with such a capacity for caring is rare. I'd only say that it may be best to have her tell Ernie, they have been best friends for years... and she may already be anxious about any possible reaction. Not that there's anything you need to worry about... but if it's coming from Hannah, and he sees how happy she is, then he'll be happy for the both of you.