You cannot meet a coach passenger at the Channel Tunnel in Folkestone—because the tunnel's passenger terminal is not in Folkestone.
The Eurotunnel Le Shuttle (vehicle/carrier train) terminal is at Folkestone (Cheriton), but this is for cars, motorcycles, coaches, and freight that drive onto the train. Passengers stay in their vehicles.
If your contact is arriving on a coach (bus) that drives onto the Shuttle, they will disembark inside their coach at the Calais (Coquelles) terminal in France. There is no pedestrian access to the Folkestone terminal platforms.
If they are arriving on the Eurostar passenger train (London–Paris/Brussels/Amsterdam), that train does not stop at Folkestone. It departs from London St Pancras International and goes directly through the tunnel.
The nearest mainline station to Folkestone is Folkestone Central (Southeastern trains from London St Pancras/Charing Cross). Ashford International (~15 mi / 25 min by train) is the major interchange for HS1 (Eurostar) and classic lines.
Don't wait at Folkestone for a tunnel arrival—you'll be waiting at the wrong place.