Short answer. Yes, it can be. Article 209 of the Revised Penal Code makes it a crime for a lawyer who has taken up your defence, or received confidential information from you in a case, to then take up the defence of the opposing party in that same case without your consent.
What the law says
having undertaken the defense of a client or having received confidential information from said client in a case, shall undertake the defense of the opposing party in the same case, without the consent of his first client
Revised Penal Code, Article 209 — Betrayal Of Trust By A Lawyer. Read the full provision →
What the second paragraph of Article 209 forbids
The offence is committed by a lawyer who, having undertaken the defense of a client or having received confidential information from said client in a case, shall undertake the defense of the opposing party in the same case, without the consent of his first client. Read the first clause carefully: it is written in the alternative. You do not have to have signed an engagement or paid a peso. It is enough that you gave the lawyer confidential information about the case — which is exactly what happens in a first consultation, even one that goes nowhere. The law protects the disclosure, not the contract. So the answer to "but he was never formally my lawyer" is that the article anticipated that argument.
Consent is the whole defence
The article criminalises the switch only without the consent of his first client. That consent is yours to give or refuse, and it must be real — informed, and given by you, not assumed from your silence or from the fact that the first matter has ended. Nothing in the article requires you to give a reason for refusing. What the article does not reach is the lawyer who acts against you in some unrelated, later dispute; the text is confined to the same case. Broader conflicts of interest across different matters are still professional misconduct and can be complained of, but they are dealt with by the disciplinary rules rather than by this criminal provision.
What the offence carries today
The penalty is prisión correccional in its minimum period, or a fine ranging from Forty thousand pesos (₱40,000) to Two hundred thousand pesos (₱200,000), or both. Those amounts are the ones fixed by Republic Act No. 10951 (2017), which revised the fines and property values throughout the Revised Penal Code; older reproductions still print the far smaller 1930 figures, so check which version any source you consult is reproducing. The article opens by imposing this penalty In addition to the proper administrative action. That phrasing matters: the criminal case does not replace the disciplinary route, and the disciplinary route does not absorb the criminal case. They run alongside each other.
The three separate things you can do
There are ordinarily three tracks, and they are independent. A criminal complaint under this article is filed with the prosecutor. A disciplinary complaint goes through the machinery that leads to the Supreme Court, which alone can suspend or disbar a lawyer. And within the case itself, a party can move to have conflicted counsel disqualified from appearing — often the most urgent step, because it protects the proceeding you are still fighting. Nothing here promises any particular result; whether the elements are made out turns on what you can show. So preserve the evidence now: your messages, receipts, the notes of your consultation, and the court record showing when the lawyer entered an appearance for the other side.