It was also likely that agents at LGA were aware that AC was arranging a "rescue" flight but were not at liberty to discuss that until it was confirmed. That is a common practice for a business-heavy well-traveled route.

As others note, in IRROPS, it is also likely that a flight which is at the moment overbooked will have empty seats at the gate. People misconnect, cancel and otherwise no show, especially in bad weather systems affecting a region. It really pays to stick around.

Lastly, I have no idea what your friend paid for his ticket, but by way of example, the cancel for a refund and simply purchase another carrier was an option here. AA is selling at US$165 for later today right now. Even if that is not a straight up exchange, it may be in range and is always something to have in one's hip pocket.