Do you find it painful because she's a good friend and you love her and you feel she is in a relationship that is not good for her, and that another child with this man is a closer tie and less chance of her getting out. Do you fear her being hurt if she in the future sees him as you see him, or he does something to hurt her and yet do you also fear that she will never see him as you see him! Do you feel that she could be happier with someone else, or on her own, than with him?
All you can do is what you have said you are doing - continue to be a good friend to her, make those congratulatory noises and be there if she needs you because you are right in your attitude that it's not your place to try to make her see the things that you see, and I think you are doing the right thing by being there for her.