You are the DM.
Players who are afraid to show you their character sheet don't understand the position of the DM.
You are part storyteller, part arbiter.
You have an entire imaginary world to pick from.
If you would want to kill the PC's, you can. It does not matter what the PC's can or can't do, if you want them dead, they are.
If only by you declaring them dead.
That said, that isn't much fun. Fun (IMO) starts when the game revolves around challanges being presented to the PC's which are within their level of competence, and overcoming them.
If there is a PC who can handle everything, that's no fun. Not to the other players, not to the DM, and probably, after the initial euforia wears off, not for the player of the PC.
To be able to match the challanges to the PC's, the DM needs to understand the strengths and weaknesses of the PC's. Not to 'exploit' them, but because without that info, a DM may have an unintended TPK on his hands (ex: because he introduced a death-trap to be disabled by a rogue-type PC, while the rogue-like PC is actually a ranger. oops.)
You and your friend both need to understand the game isn't DM vs PC's.
It's PC's (and possibly an NPC or two) against the rest of the world, and the DM as arbiter.
Try to have fun.