Short answer. Out of the free portion. Donations made to strangers are charged to the part of the estate the testator could have disposed of by will, not to the legitimes. If the gift overshoots that portion, the excess is reduced under the Civil Code's rules.
What the law says
Donations made to strangers shall be charged to that part of the estate of which the testator could have disposed by his last will.
Civil Code, Article 909 — Charging Donations To Shares. Read the full provision →
Two different pots, and gifts go to different ones
The article draws a clean line. A donation to a child is charged to that child's own legitime, because it is an advance on an inheritance he was going to receive. A donation to someone outside the circle of compulsory heirs has no legitime to be charged against, so it comes out of the disposable portion — the slice a person may give away freely because the law has not reserved it for anyone. Your father was entitled to be generous with that slice.
What 'stranger' means here
The word is not about affection or acquaintance. In this context a stranger is anyone who is not a compulsory heir of the donor — a friend, a neighbour, a caregiver, a charity, a distant relative outside the reserved classes. What matters is the legal relationship at the time of death, not how close the person was to your father. So a nephew or a long-time companion is generally a stranger for this article even though the family would never describe them that way.
The protection is the free portion's size, not a veto
You cannot object to the donation simply because you would rather have had the property. The claim only arises when the gift exceeds the disposable portion, and the article says that then they shall be reduced according to the rules established by this Code. Reduction claws back the excess so the compulsory heirs are made whole; the part of the gift that fits within the free portion stands. Whether there is an excess at all cannot be answered without valuing the estate and computing the legitimes.
What settles it
Get the deed of donation and its date, since the gift is measured at its value when it was made, and assemble a valuation of the estate as at death together with every other donation your father executed. Gifts to children and gifts to outsiders both figure in the computation even though they are charged to different pots. Raise the question during settlement, before signing any partition, and have the arithmetic done properly rather than arguing from the size of the gift alone.