Are the the Philippines that gullible or is it more because they want to ally with the US, they look up to the US, and when you have a large neighbor that wants your land, what choice do you have other than partner with another country.
Gullible is far too kind a word for the Philippines. They are beyond full blown stupid. Every problem they have is caused by their partnership with the US. It's because of this partnership that China must always use force to win what they start to make a point that China's will supercedes American will in Asia. China is a soft-hearted country and would quickly find it unpalatable to use force on a small and incredibly weak nation like the Philippines; we can negotiate very generously. But when they include American muscle (or what they hope is American muscle which actually turned out to be American eyes only) into the equation, they are no longer a small weak nation; now they are a target worth fighting. Bigger problem for them is that because the US is so unreliable, it only
looks like America has their back; this makes it so that China treats them like a worthy adversary (US+Philippines) when their true force (Philippines alone) actually deserves pity. They did this to themselves.
Eject the US; negotiate with China. All of their problems will be solved. Partner with the US to push China by force and they will have nothing but a growing number of insurmountable problems and humiliations because China must make an example out of them. It's thier choice if they want to be the positive example of how rewarding it is for an Asian country to dump the US for China or if they want to be the negative example for what China does to a country that thinks they can get above China by using the US.