Again, silly analogy, but this is the key to getting someone into a sensitive conversation- making it look nice and inviting. Bonus points if you make it seem like it was their idea to bring it up in the first place. Your friend knows that she’s gained 80 lbs, and she knows that you noticed it immediately. She might even be desperately afraid that you are going to bring it up, and if you do, she’ll feel attacked and possibly ashamed, and good luck having any real conversation with her from there.
In these situations I tend to ask obvious questions and use some very basic active listening skills.
You: Tell me about how things are going with Roger?
Her: Well, he’s good, he is very busy these days and we don’t see each other a lot, and sometimes things feel stressed at home.
You: So there’s some tension?
You: How are you feeling these days in general? (Maybe throw in concerns or stresses or insecurities you’ve felt lately- remember, she’s your friend and she can support you too)
Her: (echoing that she’s had stresses and insecurities)
You: Tell me more about that.
She may be feeling very vulnerable, so putting some vulnerability out there yourself can help create a safe, confessional atmosphere. Tell her (because really, we should all be doing this anyway) how much her friendship means to you, how much having her as a sounding board means to you, and how you hope she knows that you’re always there to be a sounding board for her too. Don’t expect that to have an immediate effect, but just mention it, sincerely, while getting coffee or walking around.
Sometimes, if I have a hunch someone is going through something difficult to talk about, I’ll bring up an issue that is sensitive to me (but not so sensitive as to keep the focus on myself for hours) and lay it out there first, to set the tone that it’s safe to talk about sensitive things. Nothing as obvious as pretending to mirror what you think she’s going through, but rather, just leading by example.
If you do all this and she still doesn’t mention anything, you’re done. You’ve done all you can do to create a nice comfy space, and plus you got to tell your friend how much she means to you, and that’s all you can do. As long as she’s not in physical danger, staging an intervention would only alienate her. Good luck to you.