He had to check-in before he reached security. He also had to be checked-in before the TSA would accept his bag for screening and dispatch to the UA ramp for being put into the cargo bay.

Also, I believe post-Lockerbie rules require that passengers be confirmed to be on the plane (via the gate reader recording their BP) in order for their luggage to travel with them. Now, I know the airlines sometimes play fast-and-loose with that rule for connections, by sending the bags off on earlier flights to the connecting point to ensure they make it. I have had flights delayed while bags were unloaded because the pax could not be verified and I have heard of folks not being allowed to stand-by for earlier/later flights because they have checked-bags.

So if he showed up at the gate with 45 mins to spare, he should be golden. UA does not process standbys until 20 mins or less before departure, and if they show a pax as checked-in or arriving via connection that can make the flight, they will not release their seat 45 mins early (heck, they often won't release them 4.5 mins early).

There has to be more going on here, that either the passenger or the reporter is omitting/obscuring to make it more of a "sad story" to tug at reader's heartstrings by making it look like the fault of the bad old airline.