Forum:Formula Synchron & Synchro Material

Say if my opponent controls a face-down Synchro Material and Junk Gardna in face-up defense mode. I control Formula Synchron in face-up defense mode. During my Main Phase 1 he activates Synchro Material, selects my Formula Synchron and then proceeds to use my Formula Synchron's effect to Synchro Summon Stardust Dragon. Would this even be a legal move? Since he doesn't control my formula, he cannot activate his effect, he can only use formula to synchro on his turn due to synchro material only allowing him to use the card to synchro, but not to activate the effect.