Considering Sephiroth has been manipulating the Heroes to "weaken" the Calamity for his own ends, I imagine that once he starts attacking it'll be something of a gauntlet thrown down; either they're with him or against him. Croach trying to talk him down is only going to be interpreted as trying to stall his "window" for assimilation.

Though, since Good!Seph will be hearing it, inside he'll be trying to pull harder against the intentions of the !Evil one which will make for some interesting CR afterwards!