Good for you! Its wonderful to see open-minded individuals! And your child will benefit from your open-mindedness and the exposure to people of differing beliefs.This person certainly could have a lot to offer your child.
Despite other responses, this person could influence your child to be a good, honest, open-minded person with strong morals and community-mindedness. And if this person has these qualities, s/he will not be arguing against whatever belief you choose to instill in your child. At most, s/he will teach your child to think, to question, and to make decisions for himself. And that could actually mean a greater, more thought-out commitment to whatever you are raising him to be.
But obviously, this person won't fit the traditional definition of "godparent" as s/he won't stand up at a baptism and promise to offer spiritual guidance. However, I do not consider myself religious and I wouldn't be offened by the title. Your friend may or may not.
If you are a church-goer and plan to have a religious ceremony, however, you may run into issues at your church. If so, perhaps you could tell this person how much you value him/her and ask to honor him/her with the title of Aunt/Uncle or make something up (life teacher, counselor, etc).
If you want to get cheesy or ceremonial you could hold a little "adoption" or commitment gathering.
Good luck.