While waiting in the line, someone also needs to call and see which one gets them help first.

Yes they can ask to be sent back home as a trip-in-vain. But if the goal is to get to SJU though, they shouldn't give up just yet. Even if the non-stop is full or cancelled, if they can at least get part of the way today (like to a hub like CLT or MIA), then they could possibly get the rest of the way tomorrow morning from there and not fully miss out on their SJU trip. Also, being willing to split up among flights can also give them more options, in case only one seat is available.

Either option they choose though, just in case, I would see if they could book a refundable until 6pm hotel room tonight at one of the PHL airport hotels, just in case they get stuck in PHL.