Such a decay would entail violation of both baryon number and lepton number. Experimentally, nobody has ever observed any violation of either of these conservation laws.
It's possible that these quantities are in fact nonconserved. This happens, for example, in some unified theories. Since there is no experimental evidence to support such possibilities, this is entirely speculative.
This answer neglects insanely improbable p+e to two photons which is gravitationally allowed.
"Gravitationally allowed" is not a term that people use, so we can only speculate about what was meant by this comment.
It's true that there are generic reasons to believe that conservation of baryon and lepton number may not hold in theories of quantum gravity. This is because there are no-hair theorems suggesting you can't tell how many baryons and leptons went into a black hole. But this is entirely speculative, since we don't have a theory of quantum gravity or any empirical evidence whatsoever about how quantum gravity works.
So in summary, the comment you refer to was posted by someone who wanted to sound wise and oracular, but was in fact making a misleadingly authoritative pronouncement.