To go international, except seattle and san fran thru Victoria airport, must go thru yvr. Can take short puddle jumper from Nanaimo (closer - only 30 mins from Duncan) or victoria airports, or sea plane from nanaimo harbour or helijet (both land at south terminal at yvr then need 10 min or less shuttle to main airport).
They can also go ferry route again. Nanaimo to horseshoe bay then south thru vancouver to yvr or Duke Pont (just south of nanaimo) to tsawwassen again then 20-30 drive north to yvr. Funny enough both routes will take 2 1/2 hrs.