Can an employer make each employee’s raise dependent on other employee’s raises?

My mom mentioned that she finally received a raise but she was still pissed off. I asked her why and she explained. The employees were all set to get like a 3% raise (too small but that’s not what this is about). She asked for more because of inflation and her boss named another employee and said if you take the raise you want then this other employee will lose their 3% raise. Is this illegal or just really shitty?