The MC is stupid, but it's not just the MC that's stupid. All Jindou needs to do is tell him that the new girl is trying to take away Ayumi's power and that Ayumi will die if that happens, so for all intents and purposes she wants to kill the girl he loves. It's not even a lie. Just share some actual information with him and he might at least be a bit more suspicious towards her.
But no, instead Jindou doesn't tell the MC anything and goes with stupid boob attacks.