Ruling Question on Gemini Counter

I have a Ruling question regarding Gemini Counter; which has an effect that reads:

"Change 1 face-up Gemini Monster you control to face-down Defense Position and negate the activation of an opponent's Spell Card, and destroy it."

This caused an argument between me and a friend. He declared an attack while having Evocter Chevalier, and Blazewing Butterfly in attack mode on his field; to which I chained Mirror Force. To my Mirror Force, he chained Gemini Counter; using it to flip Evocter Chevalier to face-down Deffense Position. Is that a legal move? I personally say no because he did not meet all of the requirments to play this card. Meaning he had no Spell Card activation to negate, therefore he can't play it in responce to Mirror Force. Yes he change a monster's battle position but the wording of the card's effect makes it look like the Spell Card's negation is a non-optional Effect, and since I did not play a Spell for him to negate, he can't use this card's effect (this due to the apparent ALL or NOTHING effect that the text implies). Since I did not play a Spell Card, he could not use both effects and therefore he could not play it.

On the other hand; he claims that the negation of a Spell Card is a secondary effect (a.k.a. optional) and does not mean that he can't chain it off of Mirror Force.

I'm sorry my side is longer here, but these are things I stated, and that is the only counter-argument I recived.

So I would like an impartial third party ruling to this please.
Can you chain Gemini Counter off of the activation of Mirror Force?

Hm... I need an admin on this one but...
It's not a cost to switch the monster FD. Also, both things happen at the same time. For example, Rinyan, Lightsworn Rogue's effect has sending a lightsworn monster to your deck and drawing one card happening at the same time. That means that both must be able to be fulfilled. You cannot draw a card if you weren't able to send a Lightsworn to your deck. My thoughts.

No. Simply No.

You cannot activate Gemini Counter as response to Mirror Force.

"Change 1 face-up Gemini Monster you control to face-down Defense Position and negate the activation of an opponent's Spell Card, and destroy it."

BOTH are required - Changing the position and negating the spell. If you cannot do both, then you do nothing.