The present regime for canonization comes from Divinus Perfectionis Magister, an apostolic constitution of John Paul II (25 January 1983). More detailed rules for the diocesan stages are in Sanctorum Mater (17 May 2007).
At first glance, it would seem that the present process does not allow for a non-Catholic to even be considered, since SM article 4 says:
The cause of beatification and canonization regards a Catholic who in life, in death and after death has enjoyed a reputation of holiness by living all the Christian virtues in an heroic manner; or enjoys a reputation of martyrdom because, having followed Christ more closely, he has sacrificed his life in the act of martyrdom.
Following the Code of Canon Law, "a Catholic" would be someone described by Canon 205: "Those baptized are fully in the communion of the Catholic Church on this earth who are joined with Christ in its visible structure by the bonds of the profession of faith, the sacraments, and ecclesiastical governance." This is more specific than "the Christian faithful", meaning "those who, inasmuch as they have been incorporated in Christ through baptism, have been constituted as the people of God" (Canon 204). Since C. S. Lewis was baptized as an Anglican, and did not join the Catholic Church, he counts as a member of the Christian faithful but not a Catholic.
This in itself does not exclude him from being a saint, since the Church recognizes the possibility of salvation even for someone outside the visible structures of the Church (in certain circumstances). The language in DPM is all about "followers of Christ" (Christi asseclae; qui Christi exemplum proximius secuti; qui Christum fidelitur sunt secuti). The issue, then, is not whether he really is a saint in heaven, but whether the Church would recognize him as such, given that part of her purpose is described in DPM as follows:
Faithful to the serious duty entrusted to her of teaching, sanctifying and governing the People of God, she proposes to the faithful for their imitation, veneration and invocation, men and women who are outstanding in the splendor of charity and other evangelical virtues and, after due investigations, she declares them, in the solemn act of canonization, to be Saints.
Since saints are meant to be imitated, the Church would have to be satisfied that Lewis was outstanding in virtue and doctrine. I don't think the Church would be ready to admit as an example of heroic virtue somebody who didn't join her when presented with the option. The theological test (which involves detailed examination of everything the candidate ever wrote) would surely also be difficult for a non-Catholic to pass - especially one who sometimes publicly disagreed with the Church, and who held some opinions that were distinctively Protestant, alongside others that were fairly idiosyncratic or even heterodox. So I think there are other people the Church is more likely to canonize first.
The second part of your question is about whether anyone who was not in communion with the Church has ever been canonized. I think this hasn't happened. The closest examples would be:
Individuals who gained some degree of local recognition, but who were never actually recognized by the proper authorities. Curiously, one Saint Josaphat, who had a degree of recognition, was actually Gautama Buddha (Josaphat = Bodhisattva).
Certain figures from years BC have feast days and are sometimes called saints, as well as featuring in the liturgy (see for example the Litany of the Saints). Abraham's day is 9 October, for example (in the 1749 Martyrology: memoria sancti Abrahae, Patriarchae et omnium credentium Patris).
Angels, like Saint Michael, may be called Saints but aren't in quite the same category as human believers.
Somewhere else where this might crop up is if there's anyone who one of the Eastern Catholic Churches recognized as a saint, between the Great Schism and their reunion with Rome, who was separated from the Roman Catholic Church during his or her lifetime and who remains venerated today. I don't know of an actual example but perhaps this has happened.