She's Jewish. He wasn't. 2+ decades later, he converted. (I don't know when he first intended to... these are the parents of co-workers of a friend of mine.)
Can they get a ketubah now? Should they?
It is prohibited for a man to live with his wife without ketubah. This is so even when there had been one that was lost or destroyed, such a situation requires another ketubah to be prepared prior to continuing to live together as man and wife (Shulchan Aruch, Even HaEzer 66:3). That a couple had been together without a Ketubah does not obviate the need, much less preclude the possibility, of one to be prepared for them.
It is difficult to imaging a situation where a non-Jewish spouse converted through a legitimate and competent Beis Din (or most illegitimate and incompetent ones as well) without this eventuality taken care of. Although I'm feel a bit silly speculating about a second/third hand story if the conversion has in fact already taken place, a Jewish wedding was likely preformed afterwards. If there is no known ketubah I would guess it was because they didn't realize the full significance of the document and misplaced and forgot about it. If this is the case it must be replaced as soon as possible.