I think that someone could live on that in the provinces, but they would probably struggle in Manila or Cebu. It also depends what quality of life you want your other half to have, and how often you want them to contact you.
I've been to the P.I twice this year, and was surprised at how expensive some things are. For example, toileteries are the same price or more expensive than they are in my local Asda/Tesco's. Clothes are no cheaper over there, phone credit is about the same. Sure, food and accomodation is cheaper, but most accomodation is very basic, and for 5000 a month, you are not going to getting anything like teh same luxury we have here.
My g/f needs at least 2000 pm just to keep her Blackberry on line and to pay for her text messages to me. I would guess that basic food is in the region of at least 4000 a month, add to that accomodation costs, transport, toileteries and money for a few clothes, you are soon looking at 25K pesos per month...and that is for a basic standard of living, not one which I am used to or would want to live.