When I was still Protestant and on my journey home I too used to go to both Catholic mass and Protestant (i.e. Baptist) services. My entire family would go to the Baptist church then after that we would go to the Catholic Church and mass.
Why did I stop? Because I read Scripture and saw the evil I did by attending Protestant churches as if it was OK. I saw the evil I did by misguiding my children in Baptist opinions. A house divided cannot stand. Christ founded the Catholic Church in 33 A.D. and commanded us to be of one body and mind. John Smyth founded the Baptist schism in c. 1609 – not GOD! The Bible has 73 books in it . A bisexual king has no right to take books out of Gods word. We don’t pick a church because the preacher is fun or the church has a big youth group or it has a great basket ball league or it has a game room or the Bible studies teach us what we want to hear – not what God wants to tell us.
You can’t say both sides are right because it is clear one side is wrong. Is Faith alone right or wrong? Is Marriage a Covenant between three parties or just a breakable contract between two people? The Pope is the head of the Church or the preacher is the Pope?
Your friend is following the easy gospel of health and wealth. He wants the fun of the Protestants and the truth of Jesus in His Catholic body. He cannot have both. It took me a long time to realize my error, it may take you time to show your friend the error of his way.
Today I still go to Protestant services whenever the opportunity arises. (I love to witness the truth to them) I do not take part in their rituals (services), I do not pray with them when their prayers are full of heresy (most of the time), I do not sing their songs of misguided “ME”-ology. When I leave their churches I shake the dust off my feet at the door as I leave.
You friend needs our prayers.