Your father may not receive the Eucharist before receiving absolution from a priest in the sacrament of penance. This doesn’t mean he can’t attend Mass; he can and should attend Mass, even if he isn’t disposed to receive 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, one must be in a state of grace, have made a good confession since one’s last mortal sin, believe in transubstantiation, observe the eucharistic fast, and, finally, not be under an ecclesiastical censure such as excommunication. For someone to receive the Eucharist without sanctifying grace in his soul profanes the Eucharist in a most grievous manner.
The 1983 Code of Canon Law stipulates:
A person who is conscious of a grave sin is not to . . . receive the body of the Lord without prior sacramental confession unless a grave reason is present and there is no opportunity of confessing; in this case the person is to be mindful of the obligation to make an act of perfect contrition, including the intention of confessing as soon as possible. (CIC 916)
What should your dad do? Here are the possibilities, starting with the best and most desirable:
1. He should go to confession, attend Mass, and receive the Eucharist worthily. In this case, your father would have returned to full communion with the Church and would be free to participate in the sacraments, having the benefit of sanctifying grace once again in his soul.
2. If he refuses to go to confession, he should still go to Mass, but not receive the Eucharist. While he would not be in the state of grace, he would avoid committing further grave offenses either by ignoring his Sunday obligation or by unworthy reception of Communion. Additionally, he would be opening himself up at least to hearing God’s word and would be fulfilling his Sunday obligation.
3. If he is unwilling to use option one or two, he should not go to Mass. In this case, he would continue not to fulfill his Sunday obligation, but at least he would avoid the graver offense of profaning the body and blood of the Lord, which would be the worst outcome.