From what I know/have seen about young women in their very late teens and into their twenties, many tend to be the thinnest they'll ever be just because they have metabolism working in their favor, but also because even if they weren't super skinny as mid-teens many often slim out (either because of getting past the stage of having little to do but snack with friends or because some keep growing (taller) past the mid-teens and/or because many kids/girls in particular, maybe can just have a little extra "softness" until they get closer to "genuine" maturity (mid-twenties). The kid who was already pretty slender as a teen often gets super slender in her late teens and through the twenties (and maybe beyond, depending on many things)..
On the other side of things, while "too thin" is so often in the eye of the beholder, if I thought my own son or daughter were too thin (these days) I wouldn't necessarily be thinking in terms of getting them to eat more. I'd be concerned about the possibility of substance abuse (or one kind or another). I'd say if you're really certain it isn't substance then just get her weight off your mind; but these days there are so many people who are substance abusers and whose parents would have no idea that they are. I don't know what the answer is there.
Having made the last depressing remark, I'd add that many girls/women are small-framed and less likely to eat like goats or sharks (or whatever/whoever just eats everything and anything). Having all kinds of things to do, demands on time, different types of stress (etc.) can mean not only forgetting to eat and (not being a big eater anyway) having little appetite. If she's someone who doesn't care about (or who stays away from) eating sugar - there you go, all the makings of super-thin in the twenties.
It's not for me to say you don't need to be concerned, but if SHE, herself, would like to add a few lbs I'd suggest she eat more carbs and dairy/fat daily. (As a small-framed women who didn't gain weight quickly, no matter what I ate on weekends, I can tell you it takes a real "concerted (and sustained) effort"(planned or not planned) at her age. If she's physically active and has a frame (height, shoe size) that's more average than it is super-small I'd think she could have a challenge trying to eat enough sugar, starches, fat (etc.) to accomplish weight gain.
Regardless of age, people who can only eat when/if they actually have an appetite just can't always eat more.