I am afraid your colleague is right. This practice is referred to as “rolled up holiday pay” where holiday pay is rolled up into holiday pay rather than paid to the employee when they actually take time off. This is a practice that is administratively convenient but following a ruling in the European Court last year, it is also unlawful. Guidance suggests that employers can only make rolled up holiday payments during a transitional period when they can be set off against holiday pay but the amounts must be clearly defined on payslips and contracts. The transition period is not expected to last for more than six weeks.
Casual and seasonal staff meet the 'worker' definition (meaning, for example, they are entitled to paid holidays and the minimum wage), even if they are not employees. To ensure your casual staff are workers rather than employees, it is essential that there should be no obligation to provide or accept work. This may, for example, take the form of performing a one-off task, or working under a 'zero hours' contract, which means the employer doesn't guarantee to provide work, and only pays for work actually done. You need to assess the business needs to ensure you can properly justify the level of risk associated with the relationship you need to get the work done
As a temporary worker, she does not have the right to be paid the same as permanent staff. As long as you are paying her the minimum wage, you can decide what to pay this person. She does, however, have the right to receive 5.6 weeks’ paid holiday per year so don’t try to shirk on that duty.