Um, this is totally just my opinion, but I think breast feeding affects every woman differently. Some women do find it makes them exhausted and drained. I never had that, personally. I think that the nurses haven't done your friend any favours by telling her that. She should definitely try to nurse that little man - it's good for both of them. And, if she is one of those mums who finds it incredibly exhausting to keep up their milk supply, then she can re-evaluate. Tell her to get a good multi-vitamin, or keep taking her prenatals, and get plenty of rest and fluids, and give it a good go.
All the best to her as she recovers!