In most cases, a Catholic who goes to an evangelical church and stops going to a Catholic church is still regarded as Christian. In fact, it is to my understanding that all a person needs to do to return to the Catholic Church in such a case is simply to go to confession and confess that one mingled with Protestantism. A baptized Catholic who falls away is not baptized again. In some more serious cases, a Catholic who becomes Protestant and then decides to return to the Catholic faith may be required to go through Rome.
In order for a sin to be “mortal,” three considerations must be met. Firstly, the matter has to be grave. Secondly, the person must have the full knowledge that what is being done is wrong. Thirdly, there must be consent on the part of the acting person. Apostasy is a grave matter. On the second consideration, the person likely thinks that what he is doing is for the Good, even though he is going against the Good in many respects by denying certain truths of the Faith. The person in most cases would convert with consent. So, it’s really the second consideration that is sticky. Does the person have enough true knowledge of the Catholic Faith to be considered culpable? If the person knows that the Pope is the Supreme Pontiff, the Vicar of Christ on earth, and yet says “I know this to be true, but I like Protestantism better because I feel better there,” then yes I would think the person morally culpable. However, if the person thinks “well, I’m not really sure about what the Catholic Church teaches, and while I see certain truths in it, I see greater truths in Protestantism,” then culpability would be less.
Of course, ignorance of God’s commandments does not make a person completely free from moral blame. There is a duty to strive to learn and know the commandments of God as revealed through the Gospel and instructed by the Church.