Check with an attorney
I would bet they can.
An employer could impose this rule if there was safety reason for instance around moving equipment andor they have clear policy of david on chain no crosses not star of david on chain no crosses not star of david.
For xtians if they continue to push consult an attorney.
The same as it is the same as it is the same as it is the same as it is the same as wearing crucifix for xtians if they continue to push consult an attorney.
Would it be against her culture if you were to wear it tucked under your shirt during work hours?
Granted you shouldn’t have to, given religious freedoms and all.