If he is Hindu, and not Catholic, than it gives an erroneous and scandalous impression. The real harm is done to the Hindu student - though others may be injured as well - who might now fancy that, though he be not a member of Christ’s Body, he is yet somehow sharing in the benefits of that Communion (salvation), leading him to imagine that delay in conversion is acceptable for the time being. It is important his distinctly seperate present relationship from the Body of Christ, that is, the Catholic Church, be articulated to him ; whereas, a direct participation - even somehwat removed - in the very Holy of Holies and Mysteries of our Faith implies a sharing in those mysteries. This is a false idea, for the Gospel proclaims, “Now is the acceptable time,” and further commands, “Repent, and be baptized, for the Kingdom of God is at hand.” The emphasis in on the immediate, and a clear distinction of seperate jurisdictions is made (i.e., God’s Kingdom, vs. the world’s or the devil’s). It is condemnable on the grounds of promoting, directly or indirectly, religious indifferentism.
His religion is false. It cannot give him salvation. Only the Catholic Church can, and membership is necessary. By permitting him to participate in the Holy Mass one actually reduces the possibility of impossible ignorance, for it follows that it is incumbent upon those enlisting his participation insure he have some understanding of exactly what he is participating in.
If there is sin or fault here, it belongs exclusively, in my opinion, on those competent and responsible persons who either permitted or encouraged the practice. If the intent was to further promote it, it would be even more serious. The Hindu man has a right to know what he is being asked to do, and what the Church teaches about these matters.