Actually, I wouldn't take this as a laughing matter, i.e., "oh those naughty folks who try to fool you into thinking this is a "direct" flight. I bet the 757 will indeed depart SFO at 1000, and those on the arriving A320 from MEX at 0950 who have a "through" ticket will simply be left behind, just as in any misconnection. UA makes no attempt to hold the second leg of an unrelated aircraft just because the flight number is the same. The next SFO-SEA is almost 4 hours later. At some point UA might notice the mistake of using the same flight number (which they often do on MEX-SFO-SEA) but having changed the timings of the unrelated legs and thus guaranteeing misconnects for all thorugh pax. At that point they might rebook all "through" pax on the much later SFO-SEA, but your friend might be well-advised to proactively get something workable from the start, rather than ending up in a middle E- seat when the issue is noticed, or perhaps even with a layover longer than the 4 hours.