Poor ol' ma! As someone who's dealt with a lot of underweight, mistreated horses, I can give you a bit of advice: Nutrition is so, so important.
Figuring out the balance of carbohydrates, proteins, etc. that your dear needs is difficult, but so worth it! Feed by weight, not by volume (which is a general rule anyways, but I find that a lot of people do this... and it's kind of a big deal). Also, adding supplements helps SO much! I've seen such a huge change in horses just with a couple added supplements! For her, because she's underweight right now, I'd think that a weight gain supplement was in order. And remember that nutrition has a direct link to not only (obviously) the horse's overall health and well being, but also to a horse's hooves, hair quality/growth, and attitude!
And (you probably already know this, but I cannot stress this enough – I've known people who just... didn't, because they didn't understand) keep salt available at all times! Mudpie loves the himalayan salt licks – and I love them because they don't dissolve in the rain and he can't bite chunks out of them!
And, to compile more basic, obvious rules of feeding: feed little often; if possible give her free choice hay at all times (preferably low protein grass hay). I really recommend feeding a supplementary grain/complete feed – Mudpie (even though he's only 6) gets senior feed which is excellent for him! It's been over two years and we're still trying to get his hooves healthy again – they're getting better but it's taking forever! And, quite obviously, keep fresh water available at all times. I'm sure you already know, but I just felt like blabbing it all out, so there it is.
And, some more advice (which I know you probably don't want xD): Blanket during winter... Yes, Southern California winters are EPICALLY AWESOME, but she'll still benefit from a blankie – a lightweight, waterproof one is kind of all you lucky duckies need! (Unless you're in a desert area... in which case I think it gets pretty chilly and she might need more) Aaaaand, since you're in California, you can get LMF Feeds, which are AWESOME! And you may also be able to get Elk Grove Milling Feeds, which are really good, too (Stable Mix is great stuff)!
I think that the first things you're gonna want to get done are her feet (trimmed, and possibly shod if they're really terrible), deworming (start with a mild one like Ivermectin and as you start her rotation, you can just gently increase the potency of the dewormers she receives), and, of course, teeth floating. Teeth floating is infinitely important because young horse's teeth are soft, and they develop hooks and uneven wear really easily – take care of it regularly, or it causes problems! xD And her vaccinations... GET RABIES! I know horses don't regularly get it and it's not in your area, blah blah blah... but to an unvaccinated horse, rabies has a 100% fatality rate and what a tragedy it'd be to lose your horse because she/he was lacking one vaccination... And all the regulars(:
Gosh, you're probably like "Well THIS person like to see herself type!" and you probably don't want my advice, but there it is(: I've dealt with a crapton of... well, crap when it comes to picking up the pieces and trying to fix a mistreated/neglected horse!
You totally have to keep us updated!!!!