Looked a little more into it, they weren’t actually the very first eaten but were the first (and by some accounts only) to be murdered for the purpose of eating. They both refused to eat human meat before fleeing (they’d been warned their murder was being planned), but were caught up with down the trail a few days later. Shortly after they were butchered the group (a segment of the party that had been sent ahead) that did it were harboured and fed in a Miwok village (the guides having also been Miwok I think). A grizzly tale.

Not related to what you are responding to but just as an interesting symbolic link between the two things you mentioned, iirc the first people killed, butchered and eaten by the Donner party were their native scouts, afterwards mostly erased from the story in popular telling. Just as the homesteaders first ethnically cleansed the native Americans before hunger for more land turned them cannibalistically against each other