Although the asking person didn't mention this, the statement "blessed be" is a well-established greeting under Wicca, specifically, and is distinct from other belief systems. Wicca is generally recognized as a religion under U.S. law. As such, this is a case of an employer compelling religious speech. Since members of many religions would be discouraged from practicing a different religion than their own the employer has to accommodate the objecting employee. The topic of business compelled speech is very deep, and so is the topic of religious discrimination, but in a general sense can be answered. The rest of this answer goes into more detail about why. The requirement that the employer accommodate is very broad, and is one of the only areas of labor law where the employee is favored rather than the employer. The laws in the United States broadly presume that if the employer has a problem with the exception the employer, not the employee, is doing something inappropriate.
Religious accommodation cares about what is called a good-faith practice. Basically, the employer is supposed to presume the employee requesting an exception is acting out of a legitimate need, and questions like how observant the employee really is or if the exemption is really needed by their religion are none of the employer's business. The only thing that matters is that the employee thinks they need an exemption, and an employer who isn't trying to exclude members of other religions won't care about those things. The employer is required to sit down and talk to the employee in a good-faith effort to help the employee continue to do their job. The employee should be willing to discuss the restriction they're trying to work around. Both sides should be trying to make things work.
Another important concept is what is actually required for the job. At that point the nature of the business becomes important. Although employers like to think of superficial matters like dress codes and manner of speech as being of import, the courts have not generally viewed them as essential to most jobs. For example, if you required someone to say, "god bless you" every time they answered the telephone, that might look consistent or project a certain set of values, but that would not actually be necessary unless the job was some kind of hotline customers called to receive blessings. If you're working for an oddities shop that primarily serves the community of Wicca worshipers by selling them religious supplies that is an important detail. In most other settings the employer is way out of line.
As described in the question right now, in the question posed above, the employee has grounds to sue, and in my personal opinion the employee should actually do so. These sorts of expectations are extremely inappropriate coming from any employer, and the employer has every reason to know that. No sensible person living in the United States would expect employees of any religion to act as members of a different religion, and these sorts of lawsuits are a necessary tool in preventing discrimination.