@ossobuco,
Yeah, I get their point too. That's why I asked should I send a gift or not. I don't think there is really a right answer.
If this were a "my adopted child's biological mom is getting married should I send a gift" question my answer would be "no".
But the fact in this case is that she was my friend long before there was Mo. It was really like a mother/daughter type thing. Before I was Mo's mom I was the stand in for his grandmother. She was clear eyed about us adopting Mo. It is what she wanted.
I guess if I had to liken this to any real world (other than my own real world) situation it would read something like this:
My beloved ex-daughter in law is getting remarried. We've kept in touch through mutual friends but we haven't seen each other in several years. She was an important part of my life and I still care a great deal for her. I would like to wish her well by sending a small gift but I don't want to bring up any bad memories on her happy day. Should I send a gift to her wedding or not?
I think Dear Abby would tell me to send a gift.