I'm certainly not an expert on the subject and if your friend decided to pursue the matter I would suggest she obtain legal advice. Anyways, I suggest the employer can do what they wish in regards to benefits, salaries, hours of work etc and in your friends case change suppliers for medical coverage. I expect the employer changed companies as the premiums were lower. However, in saying this I would expect that the change could possibly be considered a "change in working conditions" which could be perhaps argued that it is "wrongful dismissal" and the employee could leave her job and sue her employer for appropriate severance. They may not even have to leave the job but suggest to her employer that it is a case of wrongful dismissal.
Employers have the right to set salaries, wages, benefits, hours of work, and dismiss employees, etc and can change these when they wish. The only issue that comes up with the foregoing is that such changes can be considered wrongful dismissal and leave the company open to legal challenge by the employee. Just because you have certain benefits today it does not mean they are there for life. Other benefits that are changed can include things like cars, car allowances, vacation, bonus structure, pay grades, etc. Some of course are legislated (OT, Stat days, minimum vacation, etc). Anyways, just my take on the matter.