The timing of the prolapse can vary hugely in relation to the lambing, so I wouldn't be worrying too much because she hasn't lambed justecause she has prolapsed, if that makes sense. We are having problems with prolapses in a new batch of sheep we are contract farming for the first time for someone (600 sheep and we've had 12, which is NOT good), but we have no idea of their history at all, I would say the first one was 5 weeks ago now and we are not due to start for another weekand 2 days.
Agree re the culling afterwards, though (or keepas a pet if you prefer), you don't need the stress again, there is enough to worry about, and not pleasant for the sheep if it happens again, although they don't always. We hope ours will lamb successfully and raise their lambs, but every one is marked with a huge red P (or 9 if my husband's done it, he's dyslexic) so they go once the lambs are weaned.
Good luck!