"Toning" is a myth. There's no exercises that's going to magically make your butt or legs look slimmer or more ripped.
Weight lifting will make your leg or ass more round and full, while losing body fat will accentuate the lean body mass you slowly build up.
Arguably, nothing beats squats, deadlifts and some kind of lunges (I love walking lunges) for beating the shit out of your lower body.
Deep high bar squats for 10-15 reps will focus on your quads, stiff-leg dead lifts for 10-15 reps for your lower back, glutes and hamstrings, and 20 steps of walking lunges with long deep strides will tear you a new ass and generally sculpt your legs into somwthibg the Greeks would worship.
If your gf doesnt walk around in high heels all day, I'd also recommend throwing some kind of calf raises in at the end. People neglect their calves. A pair of well built thighs look weird if you don't also build some meaty calves for aesthetics.
Edit: I overlooked the underweight part.
Heavy compound exercises in general are great for putting on weight. Squat, deadlift, bench press, standing shoulder presses, bent over rows, pull downs, etc. Exercises that recruit a lot of muscle groups.
I'd recommend doing exercises for the whole body, not just the legs if you can convince your gf.
But the most important thing to emphasize is that if you have eating disorders or just naturally have problems gaining weight, YOU NEED TO EAT A LOT AND SLEEP A LOT. Look up diet recommendations for a "hard gainer" and try to figure out how to hit a caloric goal each day. Doesn't matter if you have to drink chocolate milk and eat cheeseburgers. You need to eat.
This post has been edited by Alternative Goose: 17 July 2018 - 07:28 PM