“Not a good reason to become a Catholic.”
No, it isn’t a good reason in itself, but if he makes a sincere confession and receives absolution his soul might be saved. God’s grace if for everyone who is repentant.
Wouldn’t it be ... deceptive? disingenuous? ... to receive the Sacrament of Reconciliation unless he accepts the binding-or-loosing authority of the Catholic Church which administers the Sacrament?
Of course, if this man believes all the Catholic Church believes, he should convert. But if he doesn’t, I think there should be some way in which he could lead a chaste life of repentance, regardless of his theology.