Short answer. You get the whole installment. Article 2025 of the Civil Code provides that when an annuity is paid in installments in advance, the entire installment that had already begun to run during the annuitant's life must be paid in full, rather than prorated by the number of days he actually lived through it.
What the law says
The income corresponding to the year in which the person enjoying it dies shall be paid in proportion to the days during which he lived
Civil Code, Article 2025 — Annuity in the Year of Death. Read the full provision →
What the law says
if the income should be paid by installments in advance, the whole amount of the installment which began to run during his life shall be paid
Civil Code, Article 2025 — Annuity in the Year of Death. Read the full provision →
The default rule is proration by days lived
Article 2025 starts from a general principle for the year in which an annuitant dies: the income corresponding to the year in which the person enjoying it dies shall be paid in proportion to the days during which he lived. Left alone, that rule would mean an annuity is counted out day by day, and only the portion covering the days the person was actually alive would be owed to his heirs. That is the baseline the article sets before it carves out the situation you are actually asking about.
Advance installments are the exception, not the default
The article then states a different rule for annuities paid in installments in advance: if the income should be paid by installments in advance, the whole amount of the installment which began to run during his life shall be paid. Once an installment has begun to run while the annuitant was still alive, proration no longer applies to that installment. The whole amount of it becomes payable, even though he did not live through every day it covers.
Why this turns on when the installment was payable
The distinction the article draws is about timing, not fairness in the abstract. An installment paid in advance is, by its nature, already due and payable at the start of its period rather than earned day by day as the period runs. Because your father was alive when that installment period began and the amount had already become payable in advance, Article 2025 treats it as fully his, and therefore fully due to his estate, instead of splitting it according to how many days into the period he lived.
What this means for your father's estate
Since the annuity in your father's case was paid in advance installments, the article's exception governs: the installment running when he died should be paid in full, not prorated down to the days he survived. Whether an amount already received before death needs to be returned, or how the payer's records should be checked to confirm the installment had in fact begun to run, are practical follow-up questions Article 2025 does not itself resolve, and would need to be worked out against the specific annuity arrangement your father had in place.