I have a question about sharing my faith with a Catholic friend who is mid-line atheist about why the saints are idolatry and about the holy trinity.
For a person who is a mid-line atheist probably the question of whether worshiping saints is idolatry is irrelevant. If a person believes that there is no supernatural reality, then all forms of worship are equally foolish, and idolatry is no more “bad” than worshipping of the one true God. However, if you define idolatry as the worship or veneration of anything other than God, then, by definition, the adoration of saints or praying through saints can be a form of idolatry by definition. Perhaps you can help your friend to see the logic in this. By the way, not all Catholics who use “saints” in their prayers are committing idolatry. You should be careful to separate outright idolatry (touching statues, worshipping Mary) from using icons or saints to help in prayers.
I would not try to convince a mid-line atheist of the trinity. I would try to convince him of God before I tried to convince him of the trinity. You should not “put the cart before the horse” If a person does not believe that there is a supernatural aspect of reality, then questions about whether God is one or three is irrelevant and could be downright confusing.