Originally Posted by hokiefyd

Road bikes typically have shorter effective top tubes (the distance from the seat to the stem), because of the longer reach of the drop bars. Putting a flat bar on a road bike often shortens the bike quite a bit. This may work well or may not -- it just depends on what she's comfortable with. A longer stem than you anticipate may be needed.

In other areas, the rim brakes (assuming it has rim brakes) will probably take short-pull brake levers. Most modern flat bar brake levers are long-pull. Short-pull are available, both new and used, but it's rather important to know the type you need.

"Road" and "mountain" drivetrains are usually pretty compatible at 9-speed and below if I recall correctly. So if it's got a 9-speed derailleur from a road group, I think it will work. More detail here.

Flat bar components (brake levers and shifters) are available for road groups if you want absolutely no worries about stuff working correctly.