Fuck that unqualified asshole of IT guy at the customer. He has no idea what he's talking about, constantly sets focus to unspecific bullshit and just wastes my time and energy. How do you deal with shit like that?!

do everything yourself, that's what I do 😉 (only got 10 months of day-to-day planned.)

Find a security hole in their setup, take over, do the thing yourself. Report it to their CTO afterwards if you feel really bold.
On a serious level, have you tried ccing the CTO or something in? That can work wonders with uncooperative people.