global warming is probably real, is probably but not certainly anthropogenic, is probably not going to have large effects on size and frequency of hurricanes and is probably not going to have large effects on sea level. It is a real problem but not, on current evidence, an impending catastrophe.
My response would depend on how this type of person reacts to attempts to control this "real problem". If they're willing to support emission control efforts, I'm not going to spend time arguing with them.
Many people making this type of argument though will follow it up by saying this means we shouldn't do anything about emissions, or that "adaptation" is the only thing we should do. I don't know if this is Friedman's position, but these people are the problem, and represent one of the fallback denialist positions as the scientific basis for doing nothing continues to be undercut.
So what to do about them? First, compare their credibility. One set of people have been arguing for 20 years, first that there was no warming, then that it was just natural, and then that the human-induced warming is minor. The other side has been arguing for 20 years that it's a significant, human-induced problem that we need to do something about. While the arguments theoretically stand or fall on their own, those of us who don't have time to acquire multiple PhDs to assess the evidence, need to assess the credibility of the people making the arguments. The "no biggie" denialists generally lack credibility. Maybe Friedman is virtually unique and has been making his argument unchanged for 20 years, but I doubt it. Instead, they're just looking for an excuse on which to justify their political position.
Second, doing nothing about emissions means the emissions will continously increase. I personally am not certain that climate change will definitively cause a catastrophe - but if we do nothing, emissions will be catastrophic. The modest effect outcome for climate change is only possible if we undertake quick actions and the climate turns out to be on the less-sensitive side. If the CO2 levels in the atmosphere triple, which they will barring emission reductions, then we've got a catastrophe. Do we wait, or do we deal with it now?
Finally, Friedman didn't define anything. How many Bangladeshis have to die from increased storm intensity with increased sea levels to make it "catastrophic", or how many African children die from reduced subsistence farming productivity? This won't convince denialists - it'll just make them mad - but it can help bring over people who are on the fence.
I think the denialists are on the run. The question is whether we can act fast enough to get reductions in place.
UPDATE: Friedman responds in the comments to this post. The people replying to him in the follow-up comments do a far better job of it than I would.