I think the answer must be yes, for there is no way one can support the troops while undermining their Commander in Chief. Partisan bickering only strengthens the hand of the enemy.
Still, I strongly suspect your friend's question is an academic one; for I simply can't imagine the American people electing Kerry to the presidency. He is completely unprincipled, lacks vision, has no integrity, no charm, zero affability, is an elitist, looks like Lurch, and is a boring stiff and, frankly, a bit of a gigolo. JMHO. (I feel entitled to criticize my senator, for whom I have never voted.)
I think if Kerry wins he will - despite all his bluster - see the Iraq thing through till the end. Then the Dems will give him credit for everything; suddenly it will be recast as a war of liberation, with Kerry and the Dems the heroes.