hello,
it is possible that she may be telling the truth, maybe at the time that she gave birth to her daughter,she's still w/ the father of her daughter, and when they (or the hospital) registered the birth,maybe they managed to do something for the daughter to carry the fathers surname.
If the relatives of this guy or the guy himself have doubts on this filipina, i guess, the best thing to do is, to speak to her directly and tell her that they are confused and wanted some explanation about whats bothering them, and so maybe she can explain it to them or to her boyfriend, for them to understand.