I'd like to emphasize here that apparently, our Warlock in question isn't actually casting "Wish". He's making a wish, that is, a REQUEST, to a specific individual entity, who has already warned him that there will be a price. The price was not specified, and although the implication is that words will be twisted, quite frankly, with an open-ended price tag like that, even the most air-tight perfectly worded wish could have profoundly bad effects... NOT EVEN RELATED TO THE WORDS YOU USE.
If I had an NPC offer a wish to my players with such a cavate, I'd be certain that the price had nothing whatsoever to do with the words the player uses to make the wish. For example, your eldritch blasts no longer harm your friends exactly in the manner you desire... but from that day forewards, you will never see sunlight again. Or all food becomes as bland as mush. Or perhaps a varient of Jadis's curse from The Magician's Nephew... every time you lie down there will be ice blocks at your feet and every time you sit down the seat will feel red hot.
That sort of thing.
The efreet said there would be A PRICE. The price was not specified! Don't think that words will help you! And I wouldn't say that this is nessessarily the DM trying to screw you guys over... if you actually learn the spell Wish, I doubt such things would happen to you if you follow the RAW saying exactly what the spell does for you. As it is, I actually think what your DM has going on here is pretty cool.