Praise be to Allah.
The church is a building in which the Christians practice their rituals, which include kufr, belief in trinity and worshipping someone other than Allaah.
Based on this, building churches or collecting donations to build them, renovate them and support them, is an serious evil action, because it involves helping to spread kufr and approving of it.
Shaykh al-Islam Ibn Taymiyah said:
Whoever believes that churches are houses of God, or that He is worshipped in them, or that what the Jews and Christians do is worship of Allaah and obedience to Him and His Messenger, or that He likes that or is pleased with it, is a kaafir, because his beliefs imply that their religion is valid, and that is kufr. If he helps them to open churches and establish their religion, and believes that this is an act of worship or obedience, then he is a kaafir, because this belief implies that their religion is valid.
Elsewhere he said:
Whoever believes that visiting the churches of the ahl al-dhimmah is a way to draw closer to Allaah is an apostate. If he is unaware that this is haraam, he must be told, and if he persists in that he becomes an apostate, because he has implicitly denied the verse in which Allaah says (interpretation of the meaning):
“Truly, the religion with Allaah is Islam”
[Aal ‘Imraan 3:19]
End quote from Mataalib Ooli al-Nuha, 6/281.
It says in Fataawa al-Lajnah al-Daa’imah (14/482):
It is not permissible for a Muslim who believes in Allaah and the Last Day to build a church or a place of worship that is not based on Islam with which Allaah sent Muhammad (peace and blessings of Allaah be upon him), because that is one of the greatest means of helping in kufr and making its symbols manifest, and Allaah says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression”
You have to remind him of Allaah and advise him to repent to Allaah, and to give up the work he is doing lest he fall into kufr and apostasy, and his good deeds come to nothing without him realizing.
Based on this, there is the fear that something worse will happen to your friend than his fasting not being accepted, namely kufr. We ask Allaah to guide him and to help him to do all that is good.
And Allaah knows best.