The more you nag and natter trying to make the person do things your way, even if you are right, the more you damage the trust necessary to support change. Nagging also sets up a power struggle. You want the person to do things your way. Even when you are right, your approach invites the person to rebel and stubbornly resist taking your good advice. The answers will give you a glimpse of the person's intent.
Chances are, the intent is legitimate.
The above questions, and those that follow, even though they are sometimes uncomfortable, encourage people to think more realistically. You, of course, know the answer. The point of this question is to get the person to think about the consequences of disordered eating. The most you can hope to do is plant seeds that may sprout in the future, sooner rather than later we hope, but there are no guarantees. Making changes, even minor ones, is hard, and giving up entrenched habits like starving, binge eating, and purging is overwhelming.