I can see what your friend's problem is: he doesn't know much about Catholicism.
Secondly, he believes Mary ...didn't need salvation"
Again, that's not true. Mary said "He who is mighty has done great things for me." She attributed all of the "great things" in her life to --- in her words--- "God my Savior."
There are two ways to save a person from a pit. One way is to pull them out after they've fallen in; the other way is to intervene to prevent them from falling in to begin with. Our Lord saves most of us the first way; he saved his mother the second way. (The Church calls it "prevenient.")
Why? Think about it: she is his only genetic link to the human race. He received his entire human nature from her. If she had any heritable defect, she would have passed it on to Him. God prevented that. Hence: prevenient grace. God can do that. God is not subject to the laws of time and space.
"Third, he believes that Mary, along with others who have died, can hear [our] prayers ..."
Those who love us --- our believing and God-loving mothers and fathers, relations, neighbors and friends --- intercede for us even in this life. Why would they be less able to do this when they are in the glorious presence of Almighty God?
Why would you need them to?
The Holy Word of God only lists Jesus Christ as an intercessor between man and God.