Short answer. Yes. A will can be revoked from abroad. Where the person revoking is domiciled outside the Philippines, a revocation done there is valid if it follows the law of the place where the will was made or the law of his domicile at the time. Done in the Philippines, it must follow this Code.
What the law says
A revocation done outside the Philippines, by a person who does not have his domicile in this country, is valid when it is done according to the law of the place where the will was made, or according to the law of the place in which the testator had his domicile at the time
Civil Code, Article 829 — Revocation Made Abroad. Read the full provision →
Revocation abroad is possible
Nothing ties the power to revoke a will to the testator's remaining in the country. Article 829 provides for exactly this situation: a revocation done outside the Philippines, by a person who does not have his domicile in this country, is valid when it is done according to the law of the place where the will was made, or according to the law of the place in which the testator had his domicile at the time. So your father, living overseas and domiciled there, can undo his Philippine will without returning, provided he does it in a way one of those laws recognises as an effective revocation.
Which law he may follow
For a revocation made abroad by someone domiciled abroad, the article offers a choice of governing laws. He may revoke in the manner allowed by the law of the place where the will was made, or in the manner allowed by the law of the place where he was domiciled at the time of the revocation. Meeting the requirements of either is enough. This flexibility spares a testator abroad from having to reproduce Philippine formalities he may have no practical access to, so long as the act counts as a valid revocation under one of the permitted systems of law.
If the revocation happens in the Philippines
The same article addresses the other case. Where the revocation takes place in this country, it is valid when it is in accordance with the provisions of this Code, that is, the Philippine rules on how a will may be revoked, whether by a subsequent instrument or by an act of destruction done with the intent to revoke. So the reference point depends on where the act of revocation is carried out and the testator's domicile, and the manner required shifts with it.
What to establish and record
Because validity turns on where he is domiciled and which law he followed, those are the facts to pin down. Establish his domicile at the time of revoking, as against mere temporary residence, and identify the law whose requirements the revocation is meant to satisfy. Then make sure the act genuinely meets that law's standard for a revocation and that there is durable proof of it, so the revocation can be shown when the Philippine will is eventually offered for probate. A lawyer can confirm the manner the chosen law demands before the step is taken.