I think the first thing you need to determine is what your friend believes about his deity. There are many different Christian theologies, each one is mutually exclusive of the others
i.e.,
Catholics believe that the priesthood is the conduit to God, protestants believe that each of us can contact God individually (which was the first heretical offence ... the Gnostics believed that the relationship was personal and sophia (or wisdom) was attained directly)
Calvinists believe in preordination, while others believe in free will.
There are some items that are pretty common, especially with those who don't subscribe to any particular religion:
God is Omnibenevelant (LOVE)The God, as depicted in the Bible, can hardly be considered a loving God. If you look at the carnage that is commanded by the God as evidence of worship, the idea of love is the furthest from mind.
Many Christians will say that Jesus changed that, but this concept has some flaws:
1) This supposes that the Father God made a mistake (and this isn't the first mistake he made)
2) Jesus himself is reputed to have said he didn't come to change the laws, but uphold them. He even criticized the Jews for not punishing their disobedient children according to the laws (the penalty was death)
3) Jesus was either a separate deity (came to correct his Father's mistakes), not a deity at all (having no effect on the Father God) or, equally culpable for the carnage in the OT (if he was a part of the triune God).
God is Omnipresent - where is he? Anything that has presence would leave evidence of its being there. There is no evidence trail of this god other than the 'personal experiences' of his adherants
God is omniscient - if your friend believes in free will, that is contrary to the concept of an all-knowing God. If the god already knows what your choices will be, he has already made plans for you making that choice (I like telling people that their god already knew I would be an atheist, so I must be doing His will).
God is omnipotent - how could an all powerful god make such a faulty creation? Why did he have to wipe it out completely at the time of the Flood (which isn't historical, according to weather patterns, geology and animal distributions)? Why would he need to sacrifice himself (in the form of his son)? If he had limitless power, he would have been able to change things without such drastic measures.
Here's a Christian website that explores some of the Omni-attributes of the Christian God ...
www.rationalchristianity.net/omni.html. Keep in mind that this is a Christian Apologist website and their aim is to refute the atheists ... but you might want to explore some of the articles and explanations on this site. You may encounter some of them as rebuttals to your position.
The
creation myth can be easily debunked by looking at the contradicting accounts in Genesis. Also, how did god make day and night prior to there being a Sun & Moon (and since when was the Moon a source of light unto itself?) The idea of an
Intelligent Designer is a little more difficult, but the existence of redundant/ineffectual organs and birth defect are indication that we were not designed to funtion as we do, but are rather products of our environment/evolutionary processes. Someone who is more science-minded could probably provide better resources for this aspect.
The list goes on ... and it all depends on what the theist feels his/her God is capable of. In reality, there is no solitary "Christian God" ... there are as many gods as there are Christians ... or at least there are as many are there are Christian sects.