Hmm, I'm not sure. Were you talking about a particular person previously? That's all I can think of.
For example, if you were talking about books, and she suggested one author, and you suggested another, then "Do you want me to look for something by your guy?" would mean "Do you want me to look for something by the author you suggested?"
"Your guy" might be used to refer to someone you know who provides a service. So for example, if you're discussing a plumber, your friend might ask you if he can use "your guy," referring to the plumber you usually hire.
Not a teacher - always forget that bit. Can I not just have a signature with it in or something?