well, there's this contemporary publication of several eye-witness accounts and a letter by a judge. I haven't read the whole thing but it does mention people biting the corpses and cutting off pieces with the intent of eating them (so they said). To say that he was entirely eaten would be exaggeration though (the truth is gruesome enough).

[...] sneet den Ruaerd een stuck vlees uyt het lyff omtrent sijn heupen, seggende voorgenomen hadt het selve dien avondt met den Chirurgyn Tichelaer noch te gaen braaden ende te eten. (p. 22)

In (my) translation:

[...] cut a piece of flesh from the steward's* body, about the hips, saying he planned to roast and eat it that night, with the surgeon Tichelaer.

*Cornelis de Witt, Johan's brother and fellow victim, was steward (i think that's the right translation) of Putten.