Hi,
I understand by your statement of having baptized your friend’s son, that you became his God parent and not that you actually baptized him. A baptism cannot be reversed. The reason for having Godparents is that if later the parents are not able to support the child in the Christian faith, the Godparents would step up and take their place in this.
It is unlikely that this would happen to both you and your friend at the same time. So I don’t see any reason for not having them as Godparents for your child.
Fr. Vincent Serpa, O.P.