Compensation and Overtime: Revocation of Holiday Pay

My question involves employment and labor law for the state of:
United Kingdom

My fiance lives in the UK and works for a company whose fiscal year starts on April 1, 2010. He booked 2 weeks of paid holiday (as he is due yearly) at the end of April into early May. He got back home today and saw that his paycheck was nearly cut in half. After checking, they've shorted his check by x dollars, and he got mail from his boss saying that "due to you not showing up for 2 shifts and not contacting me, I am forced to change your holiday pay to unpaid leave".
He actually did attempt to contact them, before the trip and during it and received no response. He told them beforehand that he would not be easily reached over here (in America) as it costs a fortune to call.
I can understand it if they fired him for not showing up for 2 shifts (long story), but can they keep him as an employee and just straight up revoke his holiday like that? This has literally screwed him over, he has just barely enough to pay rent and like 50 quid for the rest of the month leftover.

Any help is much appreciated... it sounds fishy to me. They've done illegal things in the past (such as not paying the employees for 30 mins on their shifts)