[quote=Mongol]Just because the guy’s Protestant doesn’t mean he rejects the Real Presence, or even Transubstantiation. See: Anglo-Catholics.
While it may be true that there are non-Catholic churches believe in the Real Presence, yet their masses are not seen as valid by the Catholic Church. As for Transubstantiation, AFAIK, none believe in this definition.
The Holy Eucharist is the most important of the seven sacraments because, in this and in no other sacrament, we receive the very body and blood, soul and divinity of Jesus Christ. Innumerable, precious graces come to us through the reception of Holy Communion.
Communion is an intimate encounter with Christ, in which we sacramentally receive Christ into our bodies, that we may be more completely assimilated into His.
Catholics and Communion
The Church sets out** specific guidelines** regarding how we should prepare ourselves to receive the Lord’s body and blood in Communion. To receive Communion worthily, (1) you must be in a state of grace, (2) have made a good confession since your last mortal sin, (3) believe in transubstantiation, (4) observe the Eucharistic fast, and, finally, (5) not be under an ecclesiastical censure such as excommunication.
Other Christians and Communion
"Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom we are not yet fully united are ordinarily not admitted to Communion. Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law. . . . "
Scripture is clear that partaking of the Eucharist is among the highest signs of Christian unity: “Because there is one bread, we who are many are one body, for we all partake of the one bread” (1 Cor. 10:17). For this reason, **it is normally impossible for non-Catholic Christians to receive Holy Communion, for to do so would be to proclaim a unity to exist that, regrettably, does not. **
Another reason that many non-Catholics may not ordinarily receive Communion is for their own protection, since many reject the doctrine of the Real Presence of Christ in the Eucharist. Scripture warns that it is very dangerous for one not believing in the Real Presence to receive Communion: “For any one who eats and drinks without discerning the body eats and drinks judgment upon himself. That is why many of you are weak and ill, and some have died” (1 Cor. 11:29–30).
(1) However, there are circumstances when non-Catholics may receive Communion from a Catholic priest. This is especially the case when it comes to Eastern Orthodox Christians, who share the same faith concerning the nature of the sacraments.
Christians in these churches should, of course, respect their own church’s guidelines regarding when it would be permissible for them to receive Communion in a Catholic church.
(2) The circumstances in which Protestants are permitted to receive Communion are more limited, though it is still possible for them to do so under certain specifically defined circumstances.
Canon law explains the parameters: "If the danger of death is present or other grave necessity, in the judgment of the diocesan bishop …” (CIC 844 § 4).
See more here.