How could he?
Guingona has been lost to the Dark Side. He has joined the KNP and has given his support to FPJ's bid for the presidency.
In exchange, FPJ made Guingona an adviser. Perhaps realizing that his civil society friends would not look too kindly on this latest move, Guingona has resigned his civil society positions. Guingona explained his proselytization:
"I do so mainly because almost everyone agrees that the nation must change, that the socio-economic downturn must reverse, that jobs be generated, poverty uplifted, and the cauldron of corruption be stifled by example and a consistent system, not only to prevent but also to hold accountable those responsible, no matter how low or high they may be."
Guingona lost faith in GMA; there is no reason why the same thing should not happen with FPJ. It must be hard growing old with a broken heart.