I am experiencing this same thing and what I suggested is buying the prayer book Blessed Be God on Amazon. A friend from my parish give me hers and it has saved and changed my life. I also keep hearing that you never know if you are doing the right thing until you experience doubts. I still get them but, we all experience them at some point or another. Another thing that helps me is either sitting in front of the Blessed Sacrament or watching what is is called Adorecast on YouTube. My priest, priesg, my parish family, listening to EWTN (especially Mother Angelica and Archbishop Sheen), reading St Faustina’s Diary, the lives of the Saints, the Catechism, and the Vatican II documents have all helped me. There are so many good resources to assist your friend in better understanding and defending the faith better. Once, he does all shadow of doubt will be erased.
The only reason I still have doubts is for reason I myself don’t even understand. Take care and God Bless!