7 is the mode of rolling two d6. If a resource tile produced on a 7 that tile would produce far too much, making the game unbalanced in favour of that tile. So it makes sense to not have a tile that produces on a 7 roll.

But since it is the roll that will occur most on average having nothing at all happen on a 7 roll would also be bad. So a game mechanic is introduced for a 7 roll that will compensate for the lack of production in this round and to maintain interest in the game (I need to count my cards, "Oh, no! Where will the robber go?") this helps players engaged in the game on the rolls where nothing is produced.

This is a random element though, and it can be bad to have a robber on your territory, so we have the soldiers, who are able to evict the robber from your territory, so you can get resource cards and hold onto soldiers in case someone tries to block your fields with a robber.

So the robber is a nice mechanic and adds some interest to a game, I don't think it's essential to get into a shouting match over it's use. It was your opponents first learning game (were you playing two player? Never play Catan two player) therefore she may well find the mechanic unfairly punishing her as she may not yet have groked the strategies for managing the apparent randomness. In that case I think a reasonable approach for a learning game would be to take the robber out for a few rounds to see how the game plays without it, and see if that allows her to get a better grip with controlling and understanding the game. Games are about enjoying yourself, if your friend is learning a game and not enjoying it why should she want to continue, why would she ever want to play it again?

Mechanisms are important in the game, and in the best designed games they each serve important and deliberate roles. However if one of the players finds the mechanic to get in the way of enjoying the game it is reasonable to make a concession to see if the enjoyment can be increased through some other means.