I am not a lawyer expert in citizenship affaires, I just copied the rules from a site I considered reliable. Well, it isn't and I want to apologize for that. The translation was bad.

After a better research, this is the official page of the Spanish Justice Ministry (the one that deals with citizenship) that mentions the rules to obtain Spanish nationality.

There is no difference between the page and what I posted about rules #1, 2, and 4. The 5th one is not mentioned in the official page, but probably is true, although not relevant for the specific case that is discussed here.

The problem comes precisely in rule #3. The literal translation of the official page is:

"Those born in Spain of foreign parents if neither of them has any nationality (stateless persons) or if neither of the parents' legislations confer a nationality to their children"

As your friend and her husband are not stateless persons, and as I suppose that US' legislation does bring to the baby a valid nationality, (s)he is not elegible at the moment of birth for the Spanish citizenship.

But as the co-forumer jabch mentions (and the official Webpage, too), any person born in Spain can claim the Spanish citizenship after one year of legal residency in Spain. The year has to be 'solid', I mean, 365 consecutive days (or 366 in case of a leap year). 6 months from 2002 and another 6 from 2005 are not valid. Even a one-day gap is enough to invalidate the period.

So, if the baby is correctly registered as a legal resident in Spain, one year later can ask for the Spanish citizenship.