Squats, Jump Squats, Deadlifts, Leg press, Hamstring curls, anything you do for legs she can do for legs.
Also
Circuit Training (which I have learned to enjoy)
She can do something like this
10 reps each consecutively
10 Jump Squats
10 Split Squats each side
10 Box Jumps
10 Burpees
10 Mountain Climbers
Rest 2 min between each round. Add weights or sets or reps as she progresses or to accommodate her level of fitness
It's going to depend on what she enjoys too and her level of fitness. When I started out u couldn't get me to do fast circuit training at gunpoint. I just wanted to lift weights, which works too of course.
I like to wing it with my workouts and do full body when I do circuit.
this is a workout I did after a morning run
12 Burpees
30 mountain climbers
15 Db 10lbs each jumpsquat
20 Left leg step ups Db
20 right leg step ups Db
30 sec plank
30 sec side plank
30 sec other side plank (people scoff at these I feel em)
10 box jumps
10 1 leg squat L 10lb DB's
10 1 leg squat R 10lb DB's
15 hanging leg lifts
1 min Jump Rope
25 Calve raises 10lbs Db's
3 sets short rest between (I didn't do the 2 min rests)
But I change from day to day, like on another day I'll go for heavier weights and try challenging myself by going heavier or raising rep amount. (just like a dude lol) Those are my favorite workouts
The most beneficial Assercise for me are Squats and Deads.
I am in noooo way a trainer (which would be great to have and I would definitely make faster progress if I did) and I am still a bit far from my ultimate goal, just sharing my personal experience what I've done and what I enjoy.
Good Luck to your girlfriend, maybe u could get her on the forums someday.
note: I wouldn't do only lower body!! Balance, train the whole body for faster proportioned safe results. You need upper body and core strength and fitness to be able to challenge legs efficiently and safely.