Low self-esteem is kinda common and sometimes lots of people have it. I think the important thing here is you cannot give her self-esteem, someone who has low self esteem will self-doubt all the time and having someone else saying that they're good generally won't cure the person of their self doubt.
Self esteem can be cured by self actualization usually, in order for one to see their own worth they must be happy with how they are in the world. And self actualization is something they must achieve on their own, though you can try to show them the way (if they do no budge though, there ay be little you can do!)
Getting into a sport, taking up a hobby, volunteering (maybe at an animal shelter?), these things can help a person feel like they're happy with themselves and can sometimes make them feel worth, making them have a higher self esteem.