Preclusive twat

In modern philosophy (a depressingly low-level course) today, we were discussing Descartes' Meditation IV (having spent the past month+ doing the previous Meditations). Descartes is concerned with the Problem of Evil with regard to false beliefs: how could the JCI God allow us to have false beliefs?
Stop right there!
What's that? You say God allows us to have false beliefs? You mean he allows crazy people to be systematically wrong while believing their selves to be sane and correct?
Okay then. Suppose God does exist. I'll grant you that in the context of your opposition of the universal doubt. But you just admitted that God's existence will not solve the problem of us being stuck with this doubt. You premise that he's a solution on the grounds that if God exists he wouldn't let us be fooled, yet you argue against an objection to his existence by saying that he can allow us to be fooled (limitless will).

(Yes, I'm over-simplifying, but it kept me amused for a portion of the class.)