Who has diagnosed him as autistic? When you say he can do everything that a non-autistic child can do except talk it sounds a lot like my grandson. Is this child social? Does he interact with people by smiling at them, pulling on their hand to get them to do something, playing with kids, look at books etc. My grandson is 3 and doesn't talk. He was evaluated thru the school district and they said that because he does interact and tries to communicate that he has a speech problem and is not autistic. He shows a range of emotions from extreme anger, hitting and kicking, and exuberant happiness. He can't say, I love you, but he gives hugs. His mother and 6 yo sister are demonstrative and he's learned from them how to express love. I think not showing emotion is part of being autistic. I do sometimes worry that my grandson's extreme emotion is a sign that something more is wrong.
I hope that your friend's child is not autistic and would also recommend a second opinion. I have a friend in her 60's who was diagnosed with both Alzheimer's and macular degeneration and a second opinion said that she had neither. It's been 3 years which seems to confirm that she has neither.
And if it is true that he's autistic reading up on it is helpful. I did some research when I was first concerned about my grandson and it was reas uring in some ways. I learned that autistic children do learn, but perhaps differently than most children, and are in many ways a blessing to their parents. I can just imagine, tho, that not receiving the demonstrative expressions of love would be painful. Your friend is fortunate to have you as a friend.
I would say that you just being there trying to understand and being empathic is the very best thing that you can do.