Assalamu 'alaykum sister
Well I think you could perhaps make dua and prayers for your friend for Allah to guide him in his affairs for example as 'Well said' wrote.
Also sister, i think if you make sincere dua for this person to Allah, even if it be in English, then that will be accepted by Allah, if Allah wills.
Your wishing of this person having prosperity and a good life, good health is very thoughtful and kind of you. As for praying to carry the burden of another person's sins, I'm not sure about that...
If you really care about this person, then maybe introduce him to Islam and some of the basic tenets of Islam. There are so many things in terms of belief that Christianity and Islam share, which is really beautiful I think. I mean when you come to think about it, I think there are more similarities than differences!
Just continue to make supplications for this person, and by God's Will he'll be guided to that which is good and protected from harm. And God won't bring harm to a person if they believe in God and worship Him well and lead their lives following the commandments of God as best they can.
Take care sister
Wassalam
Abbas.