I Corinthians 10:20 But I say, that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils.
The Gentiles in this verse refer to the pagans, those without the true God. They are making their sacrifices to false gods. You can’t see it as some innocent ignorance. They are sacrificing to devils, demons. We are not to have fellowship with those that do such things.
I think you want to be kind. I believe that you feel that it might be wrong to be so sure that you are right, and those that worship in another form are wrong. Paul and the Holy Spirit would disagree with you.
God is to be worshiped in spirit and truth. He hates images, icons, idols, or whatever word you might choose to use for them. They are worshipping false gods, demons.
Do not participate in these activities. While you might not want to offend your friend, you will be offending God.