Mark,
ASGCA has published two versions of the "Environmental Approach to Golf Courses" which has several case studies of specific design and management ideas to refute such arguments.
As TEPaul notes, the Long Island study (and others) show very little golf course effects compared to other land uses. Stuart Cohen has written many articles to that fact, and he is a PHD that used to work for the EPA. Google his name for more facts. Or, Google "Golf and the Environment" or similar. You will soon come up with several facts to refute your friend, all based on science.
But, for the record, in the few cases where I have been asked to compare the ecological impact of a golf course to pasture, farm, or vineyard, the golf course comes out way ahead. For instance, most golf courses use about a third the water of irrigated crops on the same land. And the runoff potential for golf courses is far less than open pasture. Often, fertilizer use is less as well.
As another example, in the US, over 80% of ALL fresh water used in this country is for crop irrigation. I was talking with an expert the other day, and he says there is no conservation movement in water use on the largest user of water in the US, but schoolkids are taught to turn off the tap when brushing their teeth, as if that will make a statistical difference! I visited Austin Golf Club yesterday, and Doug Petersan, Superintendent, mentioned that he uses only 0.6 lbs of N per 1000 SF per year whereas many clubs use that much or more per month. Whereas the turf irrigation manuals suggest that he would need to put down 3 acre feet of water per acre in Austin, he puts only 2.
I could go on, but I believe there has been a "silent" revolution going on among supers to drastically reduce all inputs, which rarely gets reported, and your friend is arguing about golf cousre practices of twenty years ago, not current ones.