S.H.
This discussion needs to take place with both the parents and the grandmother. If you don't think you will be able to care for the child don't agree to it. Let you friend choose another person who is in a position to be a parent in 10 years. A child needs someone that can be depended on to be there for him/her anything else is unfair to that sweet boy.
This is a serious decision, if you cannot fulfill the duty of raising a child then don't agree to be his parent.
Either way, give it some thought.