Originally Posted by bosboy73

I looked at the verbiage again on the Voucher. It says "Please present this Lufthansa Business Lounge Voucher together with your boarding pass for a same-day flight operated by a Lufthansa Group Carrier (Lufthansa, Austrian Airlines, Brussels Airlines, SWISS, Germanwings, Eurowings) to our lounge personnel at your departure airport." I think I answered my own question - unless the FRA lounges make unwritten exceptions for UA flights or something like that which I wouldn't know.

Sometimes some lounges make exceptions - I remember being able to use my United Club one-time pass at a Scandinavian lounge a couple of times so for whatever reason that lounge was allowing it.