Half-celestial does not make any requirements on your alignment, and you cannot fall as a half-celestial the way a paladin can; there is no such thing as an “ex-half-celestial,” no way to lose the template or its powers. They list “Always Good” as an alignment, but always does not actually mean always with alignment; you can have non-Good half-celestials. Half-celestials have an innate, almost biological pressure to be Good—that is what the Always designation means—but they still have free will and can choose to behave otherwise.
So there is nothing wrong. This is the player’s character, and they are roleplaying a person who would be a disappointment to their celestial parent, but nothing more. Note that this has nothing to do with whether or not the quality of roleplaying is high or low: that’s a purely subjective assessment, largely in the eye of the beholder, and such a character could easily be roleplayed “well” or “poorly” for any given subjective definition of those terms.
Now, if the character runs into said celestial parent, that parent might take offense at their actions, and of course society as a whole is going to treat them according to their actions, but they do not “fall,” per se.
Also note that “fallen” angels, themselves, aren’t really any different on this score, and don’t lose their powers. A fallen angel can be Evil while retaining things like the Good subtype, and their celestially-empowered supernatural abilities. Those with cleric spellcasting may lose that spellcasting; it’s a bit unclear just how those angels are specifically powered. But even if so the fallen angel may change his or her faith to regain cleric spellcasting, now powered by a dark patron.
In much the same way that the half-celestial may disappoint his or her parent, the celestial may anger their former peers who are still Good, who may see this fall as an act of betrayal. Again, the consequences for the alignment change are purely social, not mechanical.