Don't book online. Find the flights you want and book on the phone. A agent will be able to book a round trip and a one way on the same itinerary and reserve seats next to each other.
-1. Booking over the phone is often significantly more expensive (and never cheaper).
I forgot most people might pay a 5-50 dollar phone booking fee on many major airlines. However the ticket itself should never be more expensive.
You`ve made a right desicion! If she didnt try to find alternative, why then you have to bend over backwards to make for two?!