you can't tell how healthy a person is by looking at a scale. there are plenty of thin people who are much unhealthier than overweight people. please don't judge her and just *telling* (ie shaming) her isn't going to motivate her. it may make matters worse for her if you make her feel badly.

if you are truly concerned for her (and not just trying to prove her wrong) consider doing something positive for her. is she getting exercise? if not, suggest taking a walk with her. cook her a healthy meal and turn her onto new foods. just don't tell her what you're up to or you could damage her psyche. nothing feels worse than having a skinny person say "just quit eating crap and exercise." I know from experience.

please be kind to her.