Hi all, pardon me if this is a stupid question, but I'm far from an airline travel pro

I live near Halifax, Nova Scotia (YHZ) and am looking at travel between here and NYC. Partly for myself to visit my parents in NYC, and partly for them to visit me here in Halifax.

I went to Expedia, and found that nonstop tickets between the two cities are around CAN$550 or so, which is about normal.

For reasons that are too long to explain, I wound up looking at the airfare from Portland, ME (PWM) to Halifax. It showed the lowest fare as CAN$409, with a stop in.....Newark So, Portland to EWR, then EWR to Halifax.

So I guess my question is: can my parents buy that ticket from Portland to Halifax, ditch the first portion of the ticket (from Portland to Newark), then board the second part of the itinerary (from Newark to Halifax) and save themselves $150 apiece compared to a "normal" nonstop itinerary between Halifax and Newark?

If so, how does that work, exactly, since they wouldn't be boarding the flight at its origin in Portland? If they don't show up for the Portland flight to Newark, would the second part of their itinerary be cancelled? Would they be able to check in at Newark, and check their bags, normally?