They loved his idea so much they stole it.
What made them copy the idea? Why did they do that? They didn't even ask permission.
They had been curious about this idea for a while and have been watching him silently. When he left the room, they immediately discussed their plan.
It is wrong to steal someone else's work without compensation. You should report the behavior to make sure the person gets just treatment.
So what I should do, give the person credit for their work? That is just silly.
Yes, that is what definitely should happen. Whoever came up with the idea first should get all the credit, not the thieves. It's quite unethical to steal like that, and the person's life might be ruined when he doesn't get what he's entitled to.