My experiences have been pretty much from the other side.  I have a nephew (my brother's first wife's son he adopted when brother married nephew's mother.  They divorced but nephew is still part of the family) who has adopted 2 kids that way.  As I understand in his state the birth mother has 6 months to change her mind. The youngest was 6 months old last week.

Yeah, this is an issue my family has been quite familiar with for a while.

It sounds like the die has been cast.  All that is left is for time to show us if it was the right thing to do.  If you have any influence, or if they should by chance ask your opinion, I would ask you to ask them to do this:  Coldly with no emotion consider which situation will be better for the child?  If the single mom can support them financially and has the help needed to care for and raise a child then by all means the child should stay with its mother.

In my nephew's case twice young mothers have realized they were not able to provide the care for their children the child deserved, but someone else was able and willing to accept the responsibility.  I want to say it was a courageous act with the mother thinking only of the child's best interest.

Unfortunately many times we have all seen birth mothers whose real concern was not the child.  It was either not wanting the responsibility for the child and giving them up to avoid that responsibility, or (more common I'm afraid) the mother keeping the child so she qualified for assistance and not doing a good job of parenting.

In your situation I think all you can do is support whatever decision the young woman makes and if you disagree keep it to yourself. No, that does not disagree with what I said previously.  Regardless of whether you agree, when she says "This is what I think is best for my child"  your response needs to be "Great! How can we help you?"  Because in the grand scheme of things that is your role. Be supportive and a positive influence in whatever way you can.