Hi sir My sister is working in a food truck and...

A worker can file a complaint against the employer because of forcing her to perform a work that is completely different from the work agreed upon in the work contract, as the UAE Labor Law has allowed the worker to request termination of the work contract due to breach of the work contract and worker may file a complaint against the employer with the Ministry of Human Resources and Emiratisation requesting the termination of the work contract due to the employer's breach of the terms of the employment contract.
Here is the text of the UAE labor law related to your inquiry:
 Article (121)
 The worker may leave work without warning in one of the following cases:
 - If the employer violates his obligations before the worker in the articles stipulated in the contract or the law.
 - If the employer or his representative legally assaults the worker.