Response to Becky- A Radical Construction of People

Becky asked about constructing people in her blog. She gave the example of hearing about somebody, without ever having experienced the person. For the sake of making things easy to understand, let's pretend that Becky overhears people talking about a person named Mark. If Becky has never experienced Mark herself, a radical constructivist would conclude that she cannot prove that Mark exists, despite the fact that other people claim to have met him. Not only can Becky not make conclusions about Mark's personality or characteristics, but she cannot prove his physical existence in the world. However, even if she had experienced Mark, I still think a radical constructivist would argue that there is no proof that he actually exists. She has experienced Mark, so to her, he exists. But since we all construct our own realities, according to radical constructivism, this is not enough to prove that Mark exists in an objective world. Is this radical constructivism, or is it getting into solipsism, where an individual does not believe that anything exists outside of him/herself?