They become enlightened while they are still here.
There's a absolute quality to enlightenment, in that once it is attained it is,
1. retroactive, in that you can see that it was there for you all the time, and
2. ubiquitous, in that you can see that all beings have the enlightened essence already, just that it is covered temporarily. So a bodhisattva sees all beings as already enlightened.
The way I've written it here it's a cop-out I know, but there is profundity to it that isn't represented by my post. But that's just my opinion, not backed by scripture.
Unless I am quoting scripture or a recognized authority my posts are my opinions only and not "Dharma". I offer my posts for entertainment purposes only. I hope you find them either interesting or enjoyable--but don't take anything I say to heart without running it by your own teacher first!