I would say don't do anything like this if you don't really believe in it. Adult baptism (by whatever means) is a serious decision, and it a public declaration of faith. I would say its not something to be entered into (just) in order to be a Godparent.
It's really nice that your friend has asked you, and I too would be thrilled. I don't knw what denomination the service is for (CofE? Catholic? etc ....), but in the eyes of the church the role of the Godparents is to provide spiritual guidance and support for the child, though of course when people have christenings but don't really beleive in the message of Christianity, then this role gets diluted a bit, into general support and friendship.
My children have not been baptised, because I want them to be able to make that decision for themselves when they are older, I see it as a big decision which should be a declaration of real commitment to God. I myself was baptised by full immersion as an adult. Our children had a dedication ceremony. They don't have "Godparents" as such as they don't really exist in our church (Baptist) but they had my dh's brother and his wife as 'sponsors' at their dedication and as they are, like us, practicing Christians, they do fulfill the spiritual role. I would not have asked someone who wasn't already a serious Christian as I think the spiritual role is important.
I would say baptism is an important step and if it is something you are considering then go and talk to the minister about it, when he/she should be able to explain the implications of doing it. As an adult, you would almost certainly be expected to go through preparation classes of some kind before the ceremony was performed.
Good luck, whatever you decide to do.