Irratic surging idle points me to vacuum leak. I have a lot of experience with Chevy small block v8's both carb'd and fuel injected. 4.3 uses 90% the same parts as a small block v8 and is not that different. Check the vaccum lines under the air cleaner as well as the lines leading to the brake booster/master cylinder. You'll notice if something isn't where it should be. Belt tensioner should not have anything to do with the idle.
If he's dead set on replacing it, use a breaker bar on the bolt on the tensioner pulley, use the leverage from the bar to raise the pulley, move the belt out of the way, remove the tensioner's bolt and then just reverse the procedure to reinstall.