Sexist answer: Cause she's a g-
*head shot* *revive*
Real answer: If the physics is the problem, you two should pick simple characters and learn the game starting with them.
Great picks imo:







Good picks imo:












Maybe?:










I wouldn't try it...:











Bad idea for a beginner, imo:






Basically, you want to learn the game with characters that don't deviate too much from how the standard character would go about fighting in the game (Yoshi, Jiggly, Kirby, Ness, Duck&Dog, Olimar/Alph, Megaman, and Zelda come to mind as prime examples to avoid). It's also good to avoid starting off with characters that'll easily get you into bad habits, like Lil Mac (spamming smashes and Jolt Haymaker), Charizard (spamming Flare Blitz), Bowser (spamming Flying Slam), Meta Knight (spamming almost all his specials),Kirby (spamming Stone), and so on. After getting some real degrees of competence with someone simple, you can move on to other characters without falling into noob traps or getting caught up in learning the simple stuff AND their character specific stuff at the same time (learning Peach like this would be a total nightmare).
All of this is imo, though.