I assume you mean by ‘we both have half-fare tickets’ that you have the Halbtax. Or what exactly do you have? (Just a Halbtax on its own is not a valid ticket, it actually just makes other tickets cheaper. Because you have a monthly pass, that doesn’t matter for this case.) The monthly pass allows you to travel within the zones 30, 33 and 12. You took a train which passes by zone 11, which is not covered by the monthly pass, so it is correct that you were fined. It does not matter that you wanted to go to a destination in zone 12. To pass through the zone 11, which is not included in your montly pass, you need an extension ticket (Billet de proplongation) covering zone 11.