If your wife has not been out of the Philippines for at least a year then technically they can and sometimes refuse BB stamps as it is in the rules.
If you are thinking of living here for 6 months of the year for more than a couple of years then consider a 13a visa which can be applied for in the UK..
If its just a one off,then I cant see a problem with extending tourist visas.

Edit.. Seems they have changed the rules which should make things a little more consistant..

The Commissioner of Immigration Andrea D. Domingo informed the Department of Foreign Affairs on 4 June 2001 that, “All Immigration Officers at authorized ports of entry shall grant the benefits and privileges to all Balikbayan and former Filipinos included every time they enter the country to visit, regardless of their frequent travel”. This means that the above categories of persons could now avail of the Balikbayan benefits and privileges regardless of the number of times they enter the country within a year.

http://www.philippineconsulatela.org/balikbayan.htm