That's because you just see her as her though. A person. Not a number in a scale.
Don't feel bad. She will be very happy some people aren't commenting on her up and down weight loss. In that situation a positive comment can often be negative ( oooh look at all you have lost, but let's face it it will be back on again - either explicit or implied). You haven't. Lots of people will have. It was probably a relief you didn't mention it.
For a person whose weight fluctuates, the comments you get are 90% awful, disparaging dreadful comments that make you want to curl up and die.
I've been there for years. To talk to someone who just doesn't mention it is great. You did that for her.