Definitely don't treat the mum or baby as if everything is now changed. Mum will be grieving for the child she thought she had, and coming to terms with the news, and this can take a LONG time, but she will need you to be the same supportive friends she had before diagnosis ( which sounds like you are).
I am sure I am stating the obvious, but don't say things like "what a shame" etc. As a mum of a 3 year old with cp, I can say, yes it is, but I don't want ANYONE feeling sorry for ds or our family. He is a gorgeous little person who just happens to have some limitations, but then so do many many people with no special needs. Accept that her son will develop at a different rate to yours and your friends children, but don't be worried about telling her achievements in your own child. She will feel saddened for her child, but will still be delighted for you and yours.
Don't pretend his cerebral palsy doesn't exist, but don't see him only through his disability.
Get her the forms for claiming Disability Living Allowance, and any other support networks they need.
Most of all, just be there for her. Good luck, you sound lovely.
Fd, shucks .