I suspect that the reason why people are encouraging the OP to be cautious is that what the OP is proposing is creating a child for the purpose of adoption, NOT creating a child through surrogacy. Placing a child for adoption, no matter how open or how loving or how child-focused everyone is creates a loss, a wound of sorts, for that child. Now, depending on the child that loss might be HUGE and cause them many problems in life, or it might be so small that they hardly even think about it. I doubt any (or to avoid an absolute, i'll say i doubt many) adoptees feel NO loss at all. So you are creating a child who will be hurt by your decision. Yes, i suppose you could say "but the child would not have been created otherwise!" and i guess you have a point. It still doesnt seem like the best solution to the OPs friend's infertility.
So...the OP would not be a surrogate, no matter how much one can claim "well she is a surrogate only she is also the egg donor and the husband is the sperm donor!"...because frankly, she could get pg tomorrow by her husband on accident, and THAT wouldnt be a surro baby. Is it only because she would say "ok we're trying for John and Sally tonight! lets get pg!" that makes the child born of both of them, full sib to their own children, created within their marriage NOT their child? That seems to be a very fine line. Legally, their situation would be VERY different than most surrogacy arrangements, and i doubt they would find a reputable surro agency/lawyer who would touch this with a ten foot pole. At least in a gestational surrogacy the OPs friends would be protected, and the OP would NOT be able to keep the child if she decided to, but in this situation the OP would have all the rights as any other mother contemplating adoption.