I decided I want to lose 5 pounds.
You are absolutely not going to like this. You're going to try to convince me it's bad for me and that if it was a friend in my position I would absolutely view it as a bad thing and be concerned.
And that's true.
And I want to do it anyway.
What are you going to do, force feed me? Terminate again? That would be a bit extreme, given that it's not even technically underweight. But sure, go for it. You remember how that went last time. I'd definitely go into a self destructive spiral and become determined to get down to an extremely low weight.

The only thing you could say that would make me reconsider is to express personal concern. Not just the therapeutic relationship crap. To tell me you would find it genuinely upsetting and actually worry about me. That it would matter to you.
I'm not going to tell you that though, because that would be manipulative.
It's just that there's not really anyone else who'd be genuinely truly concerned. Everyone at work and most of my casual friends know I've been sick with stomach stuff and the one friend who knows is moving across the country in a week anyway...

There's really no one else in my life to worry about me like that.