is it legal to detain someone against their will when they are asking to call someone at a place of employment?

Yes, if they had a reasonable suspicion she had stolen money.

They could detain her on a reasonable suspicion. My question is: who was she asking to call?

If the detention was the equivalent of an arrest, viewed in light of all of the circumstances, the police officers should have advised her of her right to remain silent and her right to have an attorney present, commonly known as Miranda rights. If she asked to call to arrange to have an attorney present, then all questioning should have ceased, and they should have either arrested or released her at that point.

This is a complicated situation that needs to be evaluated by a criminal defense attorney if she is charged with a crime, and probably by a labor lawyer, too. If she is charged with a crime and cannot afford to hire an attorney to defend her, she should ask the judge to appoint the Public Defender when she appears in court.

In the meantime, she should not discuss this alleged theft with anyone except a lawyer who is representing her.

Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like Avvo, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.