Not the same situation but in our case, Dude would have been "sperm donor" (and possibly favorite "uncle") but the child would have been mine and MrS's (legally and emotionally). I wrote more about it here
The potential problem that I see is that people often don't know how they will react in the real life version of a theoretical scenario. So, unless steps are taken to ensure that things proceed as planned (legal documents, severance of parental rights, etc.) there is the possibility that someone changes their mind and, I imagine, the chaos could be extraordinary. (Not saying that this would
happen, just that the possibility has to be addressed.)
PS. I had friends offer to be a surrogate when my own fertility issues arose. I shied away from this...even though I could have a child that was biologically mine (egg donor) and MrS's (sperm donor) . If I were going to go that route I think I would prefer to use a surrogate that I had no other attachment/involvement with - so if things went south I wouldn't lose a friend as well as a baby.