Help him out. Here's why: Blended families can cause sticky, tricky situations for everyone, particularly children. They have to figure out how to understand, express, and regulate their young-but-very-real feelings about grown-up problems. In your daughter's case, she's got a stepdad in her home who's presumably a fine father, and a biological dad who isn't around as much but wants in. It's a natural reaction for her to be a little cautious of her birth father. After all, if he's not in the house every day, her relationship to him is a bit abstract to her.
But you're right that it's important that your daughter get to know her dad because, well, every child should know who her father is, especially if he wants to be a part of her life and he's a good guy. And although her feelings should be acknowledged, she's simply too young to make that call on her own (just like she's too young to choose her dinner every night).
That's why she's got you. Your job is to help her see she can love one father and still have enough room in her heart for the other. Human hearts are great that way, and your daughter should know this.
If, when she's older, the relationship with her biological father is unimportant to her, that's a different matter. But if her dad is good, as you say, your child will be lucky not only to have forged a bond with the man who helped create her but also to have an example in you for selflessly helping her make that connection.
Denene Millner is a mom of two, stepmom of one, and a former editor at Parenting. She's the author of ten books.