I am the wife of a former LDS convert to Catholicism. My husband studied Catholic material, went to Mass with me, asked questions and felt comfort in the Catholic Church that he never felt in all the 50 plus years of being LDS. He would try to answer my questions to him about LDS. One day I asked how any man could say he was greater than Jesus, greater than God…he did not know that Joseph Smith said that in one of his orations). My DH said, no one could ever say that and expect anyone to take him seriously and follow him as a religious leader. I then told him it was out of the mouth of Joseph Smith. He was silent for a long time. A few weeks later he declared his desire to join the Catholic Church.
There are many books, but a fairly good one is “When Mormons Call” by Isaiah Bennett, a former Catholic priest who converted to LDS for a few years and is back home in the Catholic Church now.
If your friend was a well informed Catholic who lost his faith for whatever reason, he will most likely see through the phony claims, etc. in not too long a time. However, if he was lukewarm in his faith and didn’t know his religion, it would be easier for him to stay in LDS. Most converts to LDS go in the front door, wander around, try to be good LDS, become dismayed at what they learn and out the back door.
My suggestion is to be gentle with him. If you know your religion well, read LDS material so you know what he is learning and then question him on it as I did my husband. If he is a new convert, he doesn’t know much about LDS. Don’t make him feel dumb or “stupid” for his decision to go LDS. Keep talking to him and “teaching” him about the REAL LDS beliefs versus Catholicism as often as you can. It is possible that his new friends will tell him not to associate with you. He will be encouraged to take on as many callings as possible to keep him so busy he won’t have time to associate with many outside his church acquaintances.
Good luck, be patient, loving and kind. For whatever reason, he is lost right now. Love, patience, good example will help bring him back to the flock. Hope this helps.
Love and peace,
Mom of 5