Since your wife has a partner - you - from an EU/EEA/EFTA country (basically, any country that uses the Euro currency) she may be visa exempt under the following rule (data from CIBT):
Applicants who hold a British blue coloured residency document "Residence Card
of a family member of an EEA national" or a "Permanent Residence Card" with a validity of five years
are visa exempt for a stay of up to 90 days if travelling with the
EU/EEA national
I think though you mentioned in a previous question that her residency period was only specified as four years, so perhaps she will NOT be exempted under this rule. If it turns out that your wife needs does need to a get a Schengen visa to visit Spain, then she can apply as a spouse of an EU national. The Schengen agreement as such requires the passport on which the visa to be issued need only be valid for duration of the stay / period visa will be issued for.
Passports or travel documents Documents must be valid for the total
duration of the planned stay.
Typically, however, most countries require you to have three to six months validity beyond the requested visa period for a visa to be issued. Spain's consular site doesn't specify what their requirement is, or at least I can't find it. If it's three months for Spain too, then you should be okay with the existing passport (as long as visa validity you request + 3 months is before May 2013). Also note that regardless of what the validity of the passport is, if it is an extended passport, i.e., a passport that had its validity extended rather than a new passport being issued, she will have to get it renewed.
You should also note that Philippines is on the list of countries that EU countries take longer to consider visa applications (~2-4 weeks) whereas for other cases it may be a 1-7 days. Keep this in mind for the timeframe of making your application. (From the German Embassy in UK site; again, I can't find the same on Spain's site.)
Now, if it so turns out that your wife does need a new passport either because it was extended or because it doesn't meet the validity criteria for Spain, the UK Border Agency says you can choose either option - keeping both old and new passport, or transferring the visa to the new passport:
If your old passport contains a multiple-entry visa or residence
permit, we can transfer that visa or residence permit to your new
passport when the old one expires.
You do not need to transfer your visa or residence permit to your new
passport. If you choose not to transfer your permit, you should carry
both your old and new passports when you travel to the UK - this will
prove to our immigration officers that you have the right to be in the
UK.
UPDATE: On whether a passport can be renewed more than six months before expiry date. This is a tricky one. Normally, many countries allow you to renew passports nine months to a year before the date of expiry, as they are aware that many visas have the six-month-extra-validity rule. I checked the Philippine Embassy in UK, Philippine Embassy in the US, and Philippine passport agency's site but none of them stipulate a criteria in this regard. (There's a whole LOAD of documents to send though it seems, if you do want to get it renewed.) I am guessing Philippines should be no different in allowing a renewal at least nine months before the expiry but this probably what you should call/visit and check. As a passport holder, your wife should get all the support from the embassy she can in clearing queries - that's what they're there for!
