So I got into an argument over exactly what constitutes line of sight and cover for a FUBAR deployment

I was of the mindset that as long as a unit was in/on terrain that provided cover, this was acceptable. My opponent stated that they must also have a blocked line of sight between units in cover. In particular there were two partially ruined buildings which had walls "cover" on two of the four sides- these walls did not block line of sight between the two pieces of terrain therefore, if my oppenent deploys into one of those pieces of terrain first, I cannot deploy into the second piece of terrain because it did not provide cover from the direction of my opponent's units.

For clarification:

There are two sets of hard-cover ruins right next to each other- the building on the north has walls on the north and east sides and is completely open on the south and west sides. The building on the south has walls on the south and east sides and is completely open on the north and west sides. If a unit deploys in one of the buildings, a unit deploying in the other building will have clear line of sight between the two. I was told this means I cannot deploy in the other building because it has now negated the cover that the building ruins provide. I must instead deploy on a different piece of terrain that has the line of sight blocked by a piece of cover.

I would have to reread FUBAR, but I don't think what you describe should matter. You and your opponent alternate deploying into cover until there is no empty terrain left. If you have cover from one side and not the other doesn't matter, that is part of the fun of FUBAR.

Your opponent was incorrect. LOS blocking is not the same as being in an area of cover. Many types of terrain will provide soft or hard cover, but still allow LOS to be drawn. The only requirements of FUBAR are:

• Each player may deploy only one unit into any area of cover on the table.
• The unit must fit completely within the area of terrain.
• No other units (friend or foe) may be deployed into the same area of terrain as a unit that has already deployed.