Would've made the same choice.
Supposing that Joel let them do it, there was no guarantee a cure would come out of it, and that the Fireflies would do anything good with it either. Humanity was pretty much done and I believe the Fireflies would've used the cure for their own agenda and secure their dominance. It'd be like giving them access to a nuke and shifting the balance of power in their favor which they'll no doubt pick and choose whoever they want to get the cure as long as they bend the knee. Nothing would change except for the main players.