are you just drinking and eating and taxiing around? how much would you need for that back in UK? that's roughly what you'd need in Krak.
No where near as much, clubs are free in Krakow and drinks are a 1/3 of the price..Eating out is also extremely good value..
If its just the weekend you should be fine with £300-£500 quid ;)
Or there abouts, he'd be better off if he had a Nationwide Building Society account, no charges to draw out and the rates are generally very good..