1) How to help a person who has converted to Islam but in Sunni and not Shia (that person being your friend)? How to help them towards the right path?

2.) How to sort the differences between Shia and Sunni

3) What books of hadith are most authentic of our shia

by

1) The best way is to give them book such as “ Then I was guided” to read and then later on discus with them.

Link to the copy of the I was guided
https://www.al-islam.org/then-i-was-guided-muhammad-al-tijani-al-samawi

2) The only way to sort out any differences is to have friendly and open discussions on what you share in common as well as what is unique to each school.

3) We have our four main books but I would suggest your check this book of Ayatullah RayShahri:

https://www.al-islam.org/mizan-al-hikmah-scale-wisdom