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.
There is a tradition of making a distinction between two different perspectives on the nature of emptiness: one is when emptiness is presented within a philosophical analysis of the ultimate reality of things, in which case it ought to be understood in terms of a non-affirming negative phenomena. On the other hand, when it is discussed from the point of view of experience, it should be understood more in terms of an affirming negation.