Hi,

opponent had a fusion monster and he attacked with it, i used Sakuretsu Armor, then he used de-fusion on the fusion monster. Is this correct?

Yes he can do that. Then the amount of monsters change on the field and sak is destroyed. Then he can attack with the new monsters.
He has to have the fusion monsters in his grave yard to use de-fusion.

D.D.

Your statement is contradicting the rules for the card.

You do not need to have the fusion-material monsters in the graveyard in order to legally activate De-Fusion.

Soory about that, I forgot all the card said. I will check better next time. Thanks for lmk.

D.D.