Zoma The Spirit/ Trap Monsters.

Hell there,
I was playing a friend and went to atk directly when he flipped a trap card over, Zoma The spirit a trap monster. The card text says that when activated it is SS in def mode but he said that he wasn't gonna SS it and just keep it in Atk since it's higher.
My question is, It MUST be SS when flipped because that's activating it right? Please explain.
BTW, I'm looking for Agent stuff and Worm stuff and Brionac so PM me if you have any of that.

You're correct.

Although I don't see why he doesn't want Zoma to be destroyed... I usually just run 2 of those into a quasar once I've exhausted its effect.

Yes it needs to be Special Summoned In Defense Position.

De standard move if he wants it in Atk to just summon it in your end phase.