FYI, everyone, this is what I do for a living. Trust me when I say that I know what I'm talking about.
Every group insurance policy has a plan document that governs the policy. The employer MUST follow the plan document right down the line. The plan document is the law, and the company can get in trouble with the law for not abiding by it. Both the IRS and the US DOL have their fingers in that pie, and depending on the state, some state governments too.
Among the items in that plan document will be a list of what dependents are eligible to be covered on the policy, if any. At least as of 2012, in 48 states there is NO requirement that an employer offer health insurance at all. There is NO legal requirement that ANY dependents be covered. There is NO legal requirement that domestic partners be covered. IF domestic partners are covered, the plan document will define what is required for an individual to meet the definition of a domestic partner. (Mingled funds, how long living together, same- or opposite-sex, etc.).
There are three possibilities, and in only one of them is the employer's action illegal.
1.) The fiancee has never been an eligible dependent under the plan and should not have been covered at all.
2.) The fiancee was an eligible dependent, but due to either a change in the plan document or a change in her status, she is no longer eligible.
3.) She was and is an eligible dependent and the employer just doesn't want to carry her any longer.
Obviously I cannot read the plan document to see which of these options it is. But ONLY if #3 is true, is there any illegality involved. If #1 or #2 is the case, and the odds are very much in favor of that, then it would not only be legal for the fiancee to be dropped, it would be illegal if they DIDN'T drop her. I'm not saying it never happens, but most employers are very much aware that (first) all such plans are subject to audit by the DOL and that if they force the dropping of an eligible dependent the DOL will be on them about it and (second) that it's very easy for the employee to see who is and is not eligible simply by asking for a copy of the plan document (which they MUST be given on request). So unless this is a very stupid employer (and yes, I am aware that there are some very stupid employers and also some that think the employees will neither question the decision nor check on its legality) the odds are VERY much in favor that the fiancee either never was, or is no longer, eligible to be covered. If that is the case, the employer MUST drop her.