Weight gain

UTBB has put it exactly right. No need to say anything at all.

Don't say anything, just move the conversation on. She has said what she says to make it clear to you that she knows she has put on weight, and that she does not intend to do anything about it and that is the end of the subject as far as she is concerned.

Possibly she is concerned, that given your iron self-control, you are going to start preaching to her and urging her to lose weight, which is off putting and distressing to someone in her situation and could lose you a good friend.

I shouldn't worry about what to say next time the subject arises, because it won't - unless you bring it up - and as you value this friendship you would never do that.

If she isn’t concerned it really isn’t for you to comment or react in any way.
She knows and is ok with it.
I myself have put on quite a bit of weight, I have mobility problems and get very little exercise and I don’t overeat .
I have shrunk due to osteoporosis so look very squat.
I certainly wouldn’t want friends commenting on the fact, I know without being told and am ok with it.

You don’t have to react any way really she’s told you quite clearly she knows she’s put weight on and doesn’t want to do anything about it at the moment so that’s the end of it isn’t it ?
No need for any reaction she’s not asking for advice or help just made a statement