Is this the worst idea ever? I know it might screw up my focus a little if I am constantly helping her.
It can be terrible or it can be fun. Depends on your lady friend's personality and how seriously she's doing this. Don't be a stupid gymbro when lifting with her, don't start making out in the gym. FWIW, I love lifting with my gf.
Any recommendations on how to help get a female who has never done compound movements up to speed properly?
Start with easy weight (i.e. just bar), work on flexibility/mobility, and coach her through like you would a gym bro you lift with. She won't go from nothing to even 100lb deadlift overnight.
What I am most worried about teaching is squats. Should I have her start with bodyweight squats to get the form down? I do low bar, not sure if this is typically easier or harder for girls...
Low bar is fine, just make sure she has the requisite flexibility. And learning the motion/recruitment is way more important with low bar, so don't let her add weight until she gets that part down.