I actually don't follow Ellen's reasoning at all (a first, I know!) If ultimately she plans to detach from Lynn's marriage and weight dilemmas, and accepts with serenity about the worst possible findings from this conversation, then why did she bother initiating it at all? "Oh ok--you're just having a lot of sex now and hoping he prefers your moves and cooking over other women's over the long term, even as you're gaining weight by the day? I should probably focus on myself right now anyway." Who is porky and over 30?
Yes, I already pointed out that Ellen might as well have staged this dialogue with herself. I was about to nominate a thin, securely married woman from this narrative as a better person to issue Lynn's warning, but come to think of it we do not have such a person. Lorrie/Laurie, in DC, is divorced, and we all know what Ed's "working late" means for the pregnant Rosa.