Short answer. No. Article 801 of the Civil Code states that the will of an incapable person is not validated by the supervening of capacity — so if your father lacked testamentary capacity when he signed the will, his later regaining full mental capacity does not retroactively make that will valid.
What the law says
Supervening incapacity does not invalidate an effective will, nor is the will of an incapable validated by the supervening of capacity.
Civil Code, Article 801 — Supervening Incapacity. Read the full provision →
Regaining capacity does not validate the will
Article 801 states directly that nor is the will of an incapable validated by the supervening of capacity. Testamentary capacity is assessed at the moment the will is executed — if your father lacked that capacity when he signed, the will was invalid at that moment, and nothing that happens to his mental state afterward, including a full return to capacity, changes that assessment. The word validated is doing the work there: a will made without capacity never becomes a will, and no later event re-executes it. What the situation calls for is a new will, made while capacity exists.
The reverse rule, for comparison
The same article states the mirror-image rule in the same sentence: supervening incapacity does not invalidate an effective will. If a testator was capacitated when the will was validly executed, later losing capacity does not undo that validity. Both halves of the article point to the same underlying principle: what matters is the testator's capacity at the moment of execution, not before or after.
Why the moment of execution is what counts
Fixing the relevant moment at execution, rather than allowing capacity at some other time to control, gives a single, verifiable point against which the will's validity is measured. Without this rule, a will's status could shift back and forth depending on the testator's condition on any given day after signing, which would leave every will vulnerable to being challenged or defended based on the testator's mental state at times that have nothing to do with when the document was actually made.
What this does not resolve
Article 801 settles that later-acquired capacity cannot cure a defect that existed at execution, but it does not itself establish what your father's mental state actually was when he signed, or what evidence is needed to prove incapacity at that specific moment. Those are separate, fact-intensive questions that would need to be established on their own, independent of what this article says about the legal effect of a later change in capacity. That proof centers on the time of signing itself, not on the recovery that came later — which is precisely the fact the article makes irrelevant.