Family Code of the Philippines Article 176 might explain it.
It says "Illegitimate children shall use the surname and shall be under the parental
authority of their mother, and shall be entitled to support in conformity with this Code.
However, illegitimate children may use the surname of their father if their filiation has
been expressly recognized by the father through the record of birth appearing in the
civil register, or when an admission in a public document or private handwritten
instrument is made by the father."
How come the mother will need any consent from the father if she has a sole parental authority over the child? Even DFA (Dept. of Foreign Affairs) and DSWD (Dept. of Social Welfare Development.) don't need any consent from the father when the mother is taking the child overseas. When it comes to adoption, they might ask for a father's consent but it still depends on if the father is supporting the child or not supporting at all (which will be tagged as abandonment). And it will not be an inter-country adoption, it will be a domestic adoption since you and your pretty wife will jointly adopt her illegitimate child. As stated in article 3 of DOMESTIC ADOPTION ACT OF 1998 Republic Act No. 8552, "the requirements on residency and certification of the alien's qualification to adopt in his/her country may be waived for the following: ....(iii) one who is married to a Filipino citizen and seeks to adopt jointly with his/her spouse a relative within the fourth (4th) degree of consanguinity or affinity of the Filipino spouse.
I've already asked 1 public attoney's office lawyer and 2 private lawyers and they were all saying the same thing!