I'd leave out the "good" part, or any
subjective comments you have on her
personal appearance, and just say "Molly, it's impossible not to notice that you've lost weight. You look so great, happy, in love", etc.
ALL women like it when you notice they've lost weight, and ALL women love compliments on how they look. What they don't like is when you compliment them with drool coming out of the corners of your mouth and burning, glassy let-me-bend-you-over-a-table-and-fuck-you-from-behind eyes.
If you don't trust yourself to say something without the appropriate pleasant, controlled facial expression, don't say anything at all.