I think it would look a bit as the revolt of 1173, would it be only because that was the core of Plantagenêt power in France, especially military.
Geoffroy would certainly benefit from a certain neutrality from Richard, too busy in Aquitaine and not that thrilled to help his father. You could even see Richard joining with Philipp II (in order to recover ducal castles), definitively making the ongoing war looking the the aformentioned revolt.
Now, devastate shouldn't be taken litteraly, as in "burn, rape and pillage", but rather as preventing Henry II to ensure his hold on Normandy and, more importantly for Geoffroy, Brittany.
-If the war goes as during the great revolt, you'd end by a similar status quo : Richard in Aquitaine, Geoffroy in Brittany and most probably Anjou.
-If the war goes against them, Henri II would most probably trust John with continental holdings (and giving how well he did IOTL in Ireland or France, it's going to backfire spectacularly), with Geoffroy stuck with Brittany and Richard out of Aquitaine (I'll tend to think John would have taken it instead).
-If the war goes well for Goffroy, he'll take at least Anjou and Richard would benefit from even more autonomy.
Anyway, the great winner would be Philippe II who would be delighted to see Plantagenet demesne going once again wild.