Playing Videogame Shooters left-handed, which one's better, controllers or keyboard + mouse?

My partner is left-handed and has always used right-handed setups for everything in videogames due to not knowing/having access to left-handed settings/devices.

But she struggles to aim in shooters, and she's getting frustrated. Which makes sense, her dominant hand is being used for the movement instead of the aiming. I'm right handed and I can't imagine myself ever getting used to aiming with my left hand, mouse or controller.

I want to get her a left handed setup, but I don't know what's better to start with, a left handed controller? or a left handed mouse (well, ambidextrous and set up for the left, left handed mouses are hard to find)