I never thought the word "friend" adequately explained a step-parent or step-parent like relationship. Friends are on equal terms, and step parents often have to guide and discipline. A friend would not discipline another friend.
Even though your daughter has an active father, that doesn't mean she can't have other father-figures in her life. Your boyfriend will surely be a father-figure at your house and he will of course set examples for her. It just means that when it comes to making decisions about raising her, discipline, activities, etc. you will be discussing them with her bio father, not your boyfriend.
You and her father will MAKE decisions, and your boyfriend will help support YOU in upholding those decisions. I would say your boyfriend's job is to support you. If you're not around, of course he should uphold any rules that you have set, and if there's discipline around he should refer her back to you.
Really at first you should concentrate on living together as 3 people. Do outings, get to know one another. Let her know right off the bat that he supports YOU, so she can't push him around. Also let her know she has a father, and he will always be her father, and your new boyfriend will never replace her father but will be another person for her to get to know, to hang with, to learn from, and to love.
Good luck!