How would you share the Catholic faith with a Seventh-day Adventist?

I would approach it as I would with anyone else, but the starting point is Ellen White. She is not from God; she is a false prophet. Jesus Christ came as a babe in Bethlehem and founded the Catholic Church, with bishops, popes, and cardinals right down the line, and there can only be one true Church. So I would begin by confronting the fact that a woman who is a false prophet is telling them how to live their lives, and see whether they can see through that. If they cannot see through that, they will not see through anything, and if you get no response, you shake the dust off your feet and walk away.