If he was looking for Supreme Court pronouncements a little closer to the times in question, maybe he should cite Texas v White. 1869.
The court issued its decision in Texas v. White after the war - it therefore could have no influence on (i.e., was not relevant to) the actions of the seceding States, or those of the federal government (unless you believe the folks of that era had ESP ;>). The opinion was also largely irrelevant, in terms of any effect on States considering secession in the future - the 14th Amendment had already been ratified, and whether intended or not, that amendment appears to prohibit State secession (barring any future amendment with applicable provisions),,,