According to the Dune Encyclopedia reference on 'deathstills', the common practice was to render the body for its water and then to carefully dispose of the small amount of dry remains in the desert in unmarked graves.
The deathstill's major components were two plasteel vats, one within the other, plus a heating device and condensing system. Its use was very simple. The body was placed in the inside vat and the space between the walls of the two vats was filled with maker oil. The lid, containing a pressure valve and vapor tube, was clamped on. Heat from an external source was transferred by the oil from the outer to the inner vat. Inner-vat temperatures reached over 200° C in prolonged use. The liquid in the body began to boil. The pressure buildup accelerated the process. Vapor escaped through the valve and was channeled through a coiled condenser tube. Condensate was collected and measured.
The Kitab al-Ibar says that 'a man's flesh is his own, but his water belongs to the tribe." After the distilling process, water measurement; and mingling with the tribe's waterhoard, what little residue remained was treated with utmost care and "buried in the earth to share with Shai-Hulud."
The remains of their own men were given a short religious memorial service. The remains of their enemies were disposed of ('discarded') without ceremony. In neither case were the remains eaten, indeed the rendering process (described in vivid detail and involving 'maker oil') would make them unpalatable if not downright poisonous to all but a Reverend Mother.
No memorials were held for out-freyn killed by the Fremen; their water
was simply reclaimed and the dry remains discarded.
For their own, however, the Fremen believed it necessary to conduct a
formal memorial service in order that the shade of the departed one
would leave in peace and visit no harm on the tribe. The ceremony
always took place at the rising of the sunset on the evening of the
death, after the body had been run through the deathstill under the
supervision of a Sayyadina.
So in short, no. The Fremen were not cannibals per se although they do drink the water that's contained in the bodies of their friends and foes alike. In Dune we see the banker at the banquet with Paul and his family make the same error, which Kynes cheerfully corrects.
The banker put down his fork, spoke in an angry voice: “It’s said that the Fremen scum drink the blood of their dead.”
Kynes shook his head, spoke in a lecturing tone: “Not the blood, sir. But all of a man’s water, ultimately, belongs to his people–to his tribe. It’s a necessity when you live near the Great Flat. All water’s precious there, and the human body is composed of some seventy per cent water by weight. A dead man, surely, no longer requires that water.”