If you work generally say 20 hours per week, then you are entitled to expect those to be able to work those same hours on a regular basis. I know that employers can reduce hours from time to time, if they have a reduction in demand from customers, but I am pretty sure that they cannot do this with such little notice. Regarding the cut in hours, if your colleague worked the extra 1.5 hours on an unauthorised basis, then Pizza Hut are entittled not to pay these hours, but if they are scheduled hours on a rota, then your colleague has a right to be paid for them.
You need to double check your employment rights with someone like Citizens Advise or similar, but it sounds to me like you are being treated unfairly.
Cheers Sue