1.Sweating causes some sodium loss, but because the sodium content of sweat is so low relative to body fluids, your sodium concentration will rise. And it is the concentration that is important, since this controls fluid shifts in the body. It is NOT possible, even for a "salty sweater" like Randy in these scenarios, to LOSE sodium through sweat.
2.Rather, we have demonstrated that the only possibly theoretical way that the sodium level can fall is if Randy drinks too much. If he drinks more than he sweats, then he will really be in a lot of troube. For example, if Randy took Gatorade's earlier advice and drank 40 Oz per hour (1.2 L), then his sodium levels would fall to 125mM
Anybody considering a marathon or beyond should read this interesting post on fluid and electrolyte replacement. Take the time to read through the facinating debate on this subject that follows. The Science of Sport
There are companies out there now (Enlyten, Ultima Replinisher) that market electrolyte replacement as essential for optimal performance, even more so than carbohydrate (which is absurd!)