Praise be to Allah.
With regard to this Christian who fasts but does not pray, his situation should be investigated and he should be called to Islam, because fasting without entering Islam will not benefit him. Disbelief invalidates good deeds, as Allah, may He be glorified and exalted, says (interpretation of the meaning):
“And whosoever disbelieves rejects faith, then fruitless is his work, and in the Hereafter he will be among the losers”
“But if they had joined in worship others with Allah, all that they used to do would have been of no benefit to them”
So it is essential for him, first of all, to believe in Prophet Muhammad (blessings and peace of Allah be upon him) and enter Islam. If he believes in Muhammad (blessings and peace of Allah be upon him) and enters Islam, in that case he will be required to pray, fast, give zakaah, perform Hajj, and so on. With regard to his fasting or praying when he is still a Christian, that is not valid; his prayer is invalid, his fasting is invalid, and it will not benefit him, because one of the conditions of these acts of worship is being Muslim. If he prays when he is not a Muslim and fasts when he is not a Muslim, then his act of worship is invalid.
You who are close to him should advise him and tell him about Islam, and inform him that it is essential to believe in Muhammad (blessings and peace of Allah be upon him) and that he is truly the Messenger of Allah to all people, to the jinn and mankind, and that he has to make a commitment to Islam by directing his worship sincerely to Allah alone, and by giving up the Christian belief that the Messiah is the son of God and belief in the Trinity. He should give up all of that and believe that Allah is one God, Who has no partner or associate, and that the Messiah, the son of Maryam, is the slave of Allah and His Messenger, and is not the son of God. Exalted be Allah for above that. For Allah, may He be exalted, has no partner and no child. Allah, may He be glorified and exalted, says (interpretation of the meaning):
“Say (O Muhammad (Peace be upon him)): ‘He is Allah, (the) One.
‘Allah-us-Samad (The Self-Sufficient Master, Whom all creatures need, He neither eats nor drinks).
‘He begets not, nor was He begotten;
‘And there is none co-equal or comparable unto Him.’”
If he gives up what the Christians say of this abhorrent belief, and believes in Muhammad (blessings and peace of Allah be upon him), and believes in the message he brought, and he makes a commitment to Islam, then in this case he will be a Muslim, with the same rights and duties as other Muslims; he will be required to pray and fast, to give zakaah if he has wealth, and perform Hajj if he is able, like all other Muslims.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).