God instructed us to meditate on His word, but while it is not Godly to meditate on such things as you mention in your post, this does not mean that this person can't be a Christian - although if they are reborn in Christ, it means that they are an ignorant Christian.
Php 4:8 KJV
(8) Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.
2Co 10:3-6 KJV
(3) For though we walk in the flesh, we do not war after the flesh:
(4) (For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds
(5) Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;
(6) And having in a readiness to revenge all disobedience, when your obedience is fulfilled.
Rom 12:1-2 KJV
(1) I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.
(2) And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
1Co 15:56 KJV
(56) The sting of death is sin; and the strength of sin is the law.
reminding them that these things are unGodly will only strengthen their desire; minister the gospel to them, and encourage them resist such thoughts by replacing them with thoughts of the good news of God's word for those who believe. the mental health industry may be able to drug them to the point where they no longer meditate on anything, but this will not help them overcome this problem - faith in God's word heals, while the medical industry merely treats symptoms; and they haven't followed through with any of these fantasies to the best of your knowledge, right?