You might google "Mercedes Low Fuel Light". That light can come on to indicate issues with the Evaporative system, but it was my understanding that in such cases, it FLASHES.

It would also be helpful (maybe) to know what code was set that she reset.

I have a problem with the light coming on with the gauge up so high, the same sensor drives the gauge and "low fuel light" operation, in cases where it is indicating low fuel level. It is certainly not indicating low fuel level in this case, not the way you describe it, anyway.

And there are no complete wiring diagrams for these cars - all you get is what is in the service manual, it is both incomplete and less than 100% accurate.