Resolution problems with widescreen sims 4

@BigBidou  Try setting the game to fullscreen mode first, then changing the resolution.  Sims 4 shows a lot more resolution options in fullscreen mode than in windowed.

I would also suggest using a resolution that matches the 21:9 aspect ratio of the monitor, at least if your girlfriend is going to be playing in fullscreen or borderless ("windowed fullscreen" in Sims 4) mode.  Performance is generally better when the game or computer doesn't have to do extra calculations to convert one resolution to another.