You have your opinions/thoughts/bits of advice - and so do I.
What I speak from seems to work for me...
Dude, bottom line from my perspective is it has nothing to do with you. If she has a problem that appears to be causing her health issues or she has bloated to the point of not being recognisable any longer, then by all means say something. But you have no place imho if she's just let herself go a little. She will know full well that she has. It's not like she'll be looking in the mirror every day thinking how much better she feels with these extra few kilos.
I've seen friends let themselves go. If she's unhappy or it's causing problems then by all means say something. But to say something because you feel she should be a few kilos lighter is tactless, no matter how it's done...