I have mixed feelings about this. You're quite correct in that excessive weight loss can be dangerous. Absolutely.
OTOH, she is an adult, and from what you describe, she's cognitively intact. (Although if there are concerns about her cognition, she absolutely should be checked out.) It's also true that if she does truly have an eating disorder, trying to pressure her to eat more not only isn't likely to work, it's likely to make her restrict even more.
PSA: Unexplained and unintentional weight loss in an older person is considered to be due to cancer until proven otherwise. If anyone's noticing a need to literally tighten your belt and you haven't made any changes, please see your provider ASAP.
COUNTRY, CITY, AREA, PLACE -Game 21
To obliterate your address on packaging