I would say it is coming from his gas tank. The best thing to do is to drain/clean the tank. Once you have it drained, put fresh gas in it and drain. You don't have to put much gas in there...just enough to slosh around. While you have the tank off, remove the petcock and check the screen on it to make sure there isn't grit stuck in it.
I am not sure on the Sportster, but I am pretty sure it doesn't have a fuel pump. This could cause a problem when installing a filter just because you will want to make it accessible. To install the filter, just find a small one (like for a lawn mower or import car (he will REALLY love that one)), cut the fuel line, install the filter into the carb side, measure the hose from the tank and cut off excess from it. I would wait to cut the tank side until you get the filter in place. You will want to try to keep the same slope if possible. That way, hopefully, you don't affect the fuel delivery.
Hope this helps.