Hmm. Let me rephrase the question. A Catholic says that he believes Mary had children after the birth of Jesus, however in every other way follows the rules of Catholicism, prays to Mary, obeys the wishes of the Pope save in this one point, does whatever you do for Mass and suchlike. Is this hypothetical person thrown out of the Church and declared a heretic, is he excommunicated, or simply accepted within the Church?
I think your average person would be surprised to learn how many excommunications there are, but how few the RCC actually hands out.
The RCC is aware that many of its individual members have differences with the Magisterium; there are, for example, surveys that come out every once in a while talking about the American Catholic stance on contraception. There is a surprising amount of room for noncompliance.
There is, however, something called excommunication
latae sententiae, in which a person by the very commission of an offense essentially excommunicates himself and no formal declaration about the individual is necessary. The Church merely recognizes that a person has stepped outside of communion and no longer extends to them the rights of a member of the Church. Every church does
this, I think, even if they don't have Latin words to describe it.
The Church usually makes formal declarations in the case of
scandal, where a person is intentionally taking others along with him. Does your hypothetical person make a point of obstinately or publicly diverging from the RCC? The way you've described it, this person has a private disagreement with the faith and, since they follow all the other rules, they're willing to discuss and work with someone. The Catechism defines some useful terms:
Incredulity is the neglect of revealed truth or the willful refusal to assent to it. Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him.
Heretics are not heretics because they believe, for instance, that Mary had other children. There must be a component of disrespect and unwillingness to work through the issue. A priest wouldn't call this person a heretic - he'd be thrilled to have him in his congregation. If he interrupted Mass because he had a bone to pick, however...
And: does the Pope believe this person will enter Heaven when he dies?
The Pope, if I understand his writings correctly, doesn't believe it's sufficient to follow all the rules. There's a major aspect of the faith missing there, even if a person toes every line.