[celticmusic]

The OP needs to hear the truth, it's a very succinct way of emphasizing the depth of the mistake.

[Accujack]

It's a subtle difference, but I would say a change in phrasing is what's needed.

Rather than "you are a complete idiot" it should be "You are acting like a complete idiot" or better "In this instance you've been duped/fooled, you're being taken advantage of"

[celticmusic]

that's patronizing. There's an honesty in pointing out that someone is an idiot without treating them like a child to boot.