At this late stage in pregnancy, the mare will NOT reabsorb the dead fetus.. She will get a massive infection and possibly die herself if indeed the baby is dead, It will start decomposing in her body and she will become ill herself.
Are you postive the mare is pregnant? Did you have a vet out to confirm her pregnancy? If the old owner is the one who said she is pregnant then I have my doubts, since he is not knowledgeable enough to know a mare can't reabsorb a full term baby.
Please get a vet to this mare immediately. If she is pregnant and past twelve months, something is wrong. If the foal is dead, your mare could die from massive infection from a rotting corpse inside her. You need a vet NOW>>>