My recommendation would be towards x-plane if she wants to fly jets, and towards MSFS if she wants to fly general aviation.

MSFS undoubtedly looks amazing out of the box, and that applies to places which are not hand crafted, depending on which version you buy a number of major airports are hand crafted and look even better. Unlike most other sims (x-plane, P3D) there really is limited need to buy any additional scenery, and it is of good enough quality to fly around in general aviation aircraft doing VFR flight. However, if you want to do more jet flying and airline type routes I would hold off MSFS for now. The included jets are ok but very buggy with even basic functionality around autopilot and flight computers having lots of issues. The range is also limited, from memory I think you will get a A320neo, 747-8 and 787 included. Unfortunately the arrival of third party study level aircraft is some way off and the 737 from PMDG or A320 from aerosoft are not due out until mid 2021 at the earliest - when they do come tho that will be a major plus for MSFS.

x-plane at the moment is a better choice for jet/airline type flying. There is a good range of decent level aircraft, the zibo mod for the 737 for example is near study level. And the functionality is of a high standard for these aircraft, Generally the scenery out of the box looks ok but you probably will end up spending some money on airports and possibly regional scenery too. Also the scenery management is (to my mind) one of the biggest downsides to x-plane as there is no included user friendly scenery manager - although third party ones are available like xOrganizer. Effectively what I am suggesting is with x-plane you may need to put the work in and spend some time optimising and playing with settings to get the most out of it - MSFS is far more user friendly in that respect.