Short answer. The donee gets it. Article 755 allows a donor to reserve the right to dispose of part of the donated property or a charge on it, but if he dies without having used that right, the property or amount reserved belongs to the donee. Since your father never exercised it, the donee keeps the full house.
What the law says
The right to dispose of some of the things donated, or of some amount which shall be a charge thereon, may be reserved by the donor; but if he should die without having made use of this right, the property or amount reserved shall belong to the donee.
Civil Code, Article 755 — Donor's Reserved Right To Dispose. Read the full provision →
What your father's reservation actually was
Article 755 recognizes exactly the kind of clause your father used: the right to dispose of some of the things donated, or of some amount which shall be a charge thereon, may be reserved by the donor. This let your father, despite having already donated the house, keep for himself the option to later sell part of it or pull out a sum of money as a charge against the property, without that option undoing the donation itself. The donation took effect, but the reserved slice remained under your father's control for as long as he chose to exercise it.
What happens when that right is never used
The article resolves exactly what happens if the donor dies without acting: but if he should die without having made use of this right, the property or amount reserved shall belong to the donee. Because your father passed away without ever selling the house or withdrawing any sum from it, the reserved right simply lapses with his death, and whatever it covered, the property or the amount that would have been charged against it, now belongs fully to the donee rather than falling back into your father's estate.
Why the reservation does not become an estate asset
A reserved right of this kind is personal to the donor; it exists only for him to exercise during his lifetime, and it is not treated as a separate asset his heirs can invoke on his behalf after he is gone. Since the right was never used, there is nothing left of it to pass through your father's estate. The donation to the donee, having already been complete except for that reserved possibility, becomes unqualified once the possibility disappears with your father's death.
What this means for settling your father's estate
The house, including the portion or amount your father could have taken back, should be treated as fully belonging to the donee rather than as part of your father's estate available for distribution to his heirs. If you are involved in settling the estate, this house should not be listed as an estate asset on the theory that the reserved right somehow reverted to your father's heirs, since Article 755 directs it to the donee once the donor dies without exercising it.