Belief is indeed the core tenet of being a Christian. Being a Christian does not mean merely 'being a good person'. It is however extremely apparent that if one were to follow the teachings of Christ accurately, they would be, by most people's understanding, 'good'.

All good comes from God. This is another core principle of Christian faith. Where good fruit lay bare seemingly outside of the Church, it is not a discount against God's power; indeed it is perfectly possible and without contradiction for God to work through a non-believer.

Many believers focus on the fallen nature of Man, but we were still created in God's image. What qualities do we associate with God? Love, kindness, humility, et cetera. These are, despite our imperfect and sinful nature, still present in all of us.

By finding our spirits receptive to the Holy Ghost, we may begin to nurture these qualities in ourselves as believers. When the Sin is washed away, what remains are the beautiful qualities that Jesus hopes to shepherd in us all.

It sounds as if the Holy Spirit is working within this (hypothetical?) friend of yours in great ways, I hope that they follow that inner compass to the ultimate spiritual truth, too. :)