I think someone already mentioned it, but there was this one scene, where Bartimaeus gets almost eaten by an Foliot. So, I think its possible for a djin to eat a higher-ranked spirit...
Bartimäus was weakened in this time... maybe it depends on how much power a spirit actually have in the moment, when it gets eaten? Also, in the footnote Bartimäus mentioned something about this stuff. The substance of the victim gets absorbed by the "eater"

... So... i think yes, he could eat an afrit, if its highly damaged...