Regulation 12 of the Working Time Regulations (implementing European Directive (93/104/EC), provides that where an employees working day is longer than 6 hours, those employees are entitled to a 20 minute break spent away from the workstation. Therefore, your friend is entitled to a 40 minute break away from his/her workstation during that 12 hour shift.
I also work 12 hour shifts and I get this minimum 40 minutes break away from my workstation. How we do it is quite flexible and changes between days or nightshifts. These breaks are unpaid however - I get paid 11 hours and 20 minutes each shift. This also means that when I take a paid holiday I deduct what I am paid for - I do not deduct 12 hours!!
Your friend is entitled to get at least what I get. However if he/she was getting more than this (perhaps paid), then that is a matter of contract, and your friend's employer cannot unilaterally change the terms and conditions of the contract - your friend must agree. If he/she does not agree then a grievance must be raised. Doing nothing (quite disgracefully in my opinion) implies that the employee accepts these changes. If, having exhausted the grievance process, the employer refuses to observe the agreed contract terms, the employee must resign and raise an action for breach of contract - and/or, if applicable, an unfair dismissal claim at an employment tribunal.