If your daughter had not been a serious contender, it’s unlikely that an admission official would have interviewed the alumnus who wrote the recommendation. So this interview is most likely a good sign (although it would be helpful to know who, specifically, contacted the alum and what sorts of questions were asked).
BUT … Ivy admission officials always end up “rejecting” some strong applicants whom they would really like to admit. So just because your daughter probably scaled some preliminary hurdles, it doesn’t necessarily mean that she’ll get good news this week.
But … fingers crossed … she will.