I would be very wary of doing something so complicated with a friend. I'm sure she has the very best of intentions but you never know how things are going to work out in the end. What if it turns out to be a very difficult pregnancy and she ends up resenting it? What if you turn out to have very different views on antenatal testing and care during pregnancy? What if she wants to keep the baby after all? Any number of things could go wrong.
When it comes to something as important as this, I would always prefer a relative stranger over a friend or relative. I think it is far easier to come to a clear-cut agreement with someone who doesn't have such a close relationship with you. Dylan's birth mum has already given up children for adoption so she knows what she is getting into, so I would see her as a far lower risk. Also you would have the genetic connection to Dylan.
You are very lucky to have such a generous friend, but I would be inclined to decline her offer and stick with Dylan's birth mother.