Well, two thing. Firstly by law breaks need to be a minimum of 20 minutes when an employee works 6 hours or more, so 15 minutes wouldn't be allowed.
But regardless of that, the employee doesn't get to choose the length of their breaks. As long as they meet the minimum legal requirements, the employer can certainly insist on 30 minutes if they want to, and that would be a far more responsible thing to do anyway. Even if 15 minutes met the legal minimum, the employer would certainly be within their rights to say no to that.