What are the chances of a Phillipino citizen getting a change of status if they come over on a Visit (B1/B2) visa and unexpectedly decide to marry ?

It will depend upon the exact circumstances. If the alien can establish he/she did not intend to marry at the time of entry, the application for adjustment of status should be approved assuming he/she can establish the marriage was entered in good faith. If, however, the alien intended to marry at the time of entry, he/she could be denied adjustment of status, found to have made a material representation, and removed from the country. You can read more about adjustment of status at http://myattorneyusa.com/adjustment-of-immigration-status.