Tying into an existing heat run is often done, but sometimes with mixed results. Marginal examples I've seen in basements usually tap in at ceiling height instead of down at floor level (to save a few nickels on sheet metal), and often have forgotten to include a cold air return. Most big box stores sell adequate off-the-shelf sheet metal products, including register grills that open and close, to help your Dad. If the existing plenums serve a lot of upstairs rooms, there's the possibility that the farthest away from the furnace will feel the result of less available heat and additional "junk air" in the supply plenum. If the furnace is working at capacity now, possibly an alternate heat source should be considered, such as a baseboard heater or two. Might be worthwhile to have a qualified heating contractor take a look at things, picking his brain for the best approach to use (either using his services or DIY).
If the new room is going to be used as a bedroom, don't forget that adequate egress is required. Also, tell Pop he needs to adequately insulate the new room--walls, ceiling and floor (if possible).