Without being a personal expert, I can say that your friend's view is outside the scientific consensus. See e.g. http://www.aaas.org/news/press_room/climate_change/mtg_200702/aaas_climate_statement.pdf
which begins, "The scientific evidence is clear: global cli- mate change caused by human activities is occurring now, and it is a growing threat to society." The AAAS pretty much defines the scientific consensus, but a simple Google search will find similar statements from more or less every other relevent scientific society in the world.
Here is the American Geophysical Union, endorsed by one of my societies, the American Astronomy Society and the umbrella organization, the American Institute of Physics: "Human activities are increasingly altering the Earth's climate. These effects add to natural influences that have been present over Earth's history. Scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed during the second half of the 20th century."
Here is the American Physical Society: "Emissions of greenhouse gases from human activities are changing the atmosphere in ways that affect the Earth's climate. Greenhouse gases include carbon dioxide as well as methane, nitrous oxide and other gases. They are emitted from fossil fuel combustion and a range of industrial and agricultural processes.
The evidence is incontrovertible: Global warming is occurring.
If no mitigating actions are taken, significant disruptions in the Earth’s physical and ecological systems, social systems, security and human health are likely to occur. We must reduce emissions of greenhouse gases beginning now."
These are just the scientific organizations which I am a member of. There are many others that have similar statements.
I have participated to some extent on this sort of consensus panel (the astronomy society decadal survey), and I can tell you that they really are consensus. Nothing that is remotely scientifically controversal gets into them. If these organizations put this out as a consensus statement, that means that the leading experts in the field are uniformly confident that these statements do represent the current scientific understanding.
Of course, this does not guarantee that they are correct. But I would be leary of challenging them without some personal expertise. In particular, nonscientists such as politicians look foolish to me saying that they believe that the science isn't in yet. The scientists are saying as loudly as they can that the science is in. Any scientists who do not ascribe to these views have, almost by definition, fringe views. This does not mean that they are not correct of course, but does mean that their views are not shared by the vast majority of their colleagues who are experts in this.
Unfortunately, the same cannot be said of politicians and other opinion leaders, who, despite their evident lack of expertise, feel perfectly comfortable challenging the clear scientific consensus on this scientific component of this issue. I wish they would focus on the numerous political aspects of climate change.