We need to take a train from Boppard to Frankfurt airport. What is the name of the station in Frankfurt airport to get to a Lufthansa flight?
Frankfurt(Main) Flughafen. Depending if you arrive on an IC or ICE train or a regional train, it will be the Frankfurt(Main) Flughafen Fernbahnhof (long distance train station) or the Frankfurt(Main) Flughafen Regionalbahnhof.
Your best bet from Boppard to the Frankfurt airport is a direct (no changes) RE that runs every two hours at 9:16, 11:16, etc. It goes through, but doesn't require a change of trains in Mainz Hbf. It takes only 1H18m and brings you to the closer-in Regionalbhf. In between those trains, there might be some connections with changes that will get you to the airport sooner, but they often take ½ hour longer and, if you change to an ICE, get you into the Fernbahnhof, which is a longer walk to the terminals.