The definition of Sexual Harassment found by Ilsi has actually changed my mind.
In its broadest definition:
Sexual harassment is bullying or coercion of a sexual nature,
Yes, in this instance, a manager using his position of authority and power to reprimand a female employee for having given out her phone number to a male suitor, and then preventing that male suitor from speaking to that employee (or giving her a message). Yes, I think that qualifies as sexual harassment in the broadest definition (and I'm not even talking about the highly inappropriate question he asked her).
That being said, that same wikipedia page goes on to say that:
The legal and social understanding of sexual harassment, however, varies by culture.
So that makes the question much trickier since we do not even know which jurisdiction or which culture this is in.
So if what happened doesn't constitute sexual harassment under the legal definition of the term, I think that the OP should consider looking at the definition of sexual discrimination in her jurisdiction. Or if it's in Europe, I think she might want to consider it a violation of her right to a private life under Article 8 of the Europe Convention Human Rights Act.
Because we have five potential issues here:
The right to privacy about your personal sexual life.
The right to give out your personal phone number to a customer.
The right to talk to someone who comes to your place of business to personally talk to you (or to give you a message).
And assuming the manager would have no problem if a family member or if a woman had come talk to her, more specifically, the question is whether the employee has the right to talk (or receive a message) from a potential male suitor at her workplace. Is that not allowed?
Or perhaps, an even better question, would a male employee be allowed to speak to a woman who comes to see him at his place of business regarding a personal matter. Will the manager interfere and ask if the male employee has been "intimate" with that woman? Or are these questions only reserved for his female employees? Hence, if she's treated differently because she's a woman, that is why I think it could be considered sexual discrimination.
Again, I believe that the jurisdiction you're in would be useful to know. Also, you should probably take a look at your employee handbook and code of conduct just in case.
Also, those issues are compounded by the fact that you're not the woman in question, you're the witness. How did the woman feel about the manager's behavior?