Fuck that bitch and fuck her fiance too. This is so incredibly selfish of them to even think it's appropriate to do, much less actually follow through with it.

You given them no indications you're interested in losing weight, unhappy with your weight, or are remotely uncomfortable with your body. That means your weight is a discussion for you and your Dr and no one else. To do this shows a lot about how they view you not fitting their "aesthetic" and how little your opinion matters in it. There's a reason they presented all this shit in a "we're doing you a favor" manner as if they'd been had all this planned out to the letter. It's fucking gross on their part.

I definitely wouldn't want to be involved in her wedding at all either OP. If it was me I'd just stop contacting her at all and peace-out, but if you want to reply I'd send a text or something telling her how that call made you feel and why you want to step down as MoH (or decline to attend at all) and just leave it at that.