I would leave the mother and her baby at peace,

Birds do puff up while laying on eggs and she may pretend to be injured when humans walk by the nest to protect her baby, BorderColliez if your friend has already gone near the nest the mother MIGHT be pretending to be injured but then again if your friend is not good enough at looking for sick birds this mother bird just might be puffed up very much to keep the egg warm.

If your friend finds the mother bird not moving or flying very well, like you would be able to grab her and pick her up then there's a reason to call a wild life centre (Even they would tell you to leave the nest alone), there's no need to hatch the egg if it's not already been hatched by the mother bird.